The measurement taken from inside the emptied cupboard
The coats are out, the meter is exposed, and there is a tape running from the corner where the pan is supposed to stand to the base of the soil stack behind the kitchen units. Two readings come off it and nothing else on this job matters as much. The first is the developed length of the route a pipe could actually take, following walls and going round the newel post rather than crossing the hall diagonally. The second is the vertical difference between the underside of the pan's outlet and the invert of the connection you are permitted to make into the stack or the drain.
Divide the second by the first and you have the gradient the building is offering. Everything after that is a comparison between that number and the minimum the adopted plumbing code will accept, and the comparison usually goes one of three ways: comfortably yes, comfortably no, or the answer that costs the most to get wrong, which is yes by two or three millimetres across nine metres of joisted floor with three bends still to be added.
Work in that order deliberately. Homeowners arrive at this decision from the catalogue end, having read about a unit that fits behind the pan and promises the loo can go anywhere. Follow them there and you install a maintained mechanical device, with its own circuit and a service interval, on a route where a hundred-millimetre pipe would have run downhill for free.
What gravity costs in height
Minimum slope is set by the adopted plumbing code and the size break moves between codes. In International Plumbing Code jurisdictions the sanitary drainage chapter works to a quarter of an inch per foot on the smaller sizes and an eighth of an inch per foot once the pipe gets larger, with the boundary between them stated in the table; the Uniform Plumbing Code sets its own break and permits the gentler figure on larger pipe only where it is approved. In England and Wales the equivalent bands sit in Approved Document H and in BS EN 12056-2, which gives both a minimum and a maximum. Read the edition in force in the jurisdiction the inspection will happen in, not the one you last worked to.
The arithmetic is unforgiving in a small house because the run is long relative to the height available. A hundred-millimetre WC branch taken from an under-stairs cupboard to a stack in the far corner is frequently eight to twelve metres of developed length once it has gone round the stair. At an eighth of an inch per foot that is between eighty-five and one hundred and twenty-five millimetres of drop before a single fitting is counted, and a suspended timber floor over a habitable room simply may not have it.
There is a ceiling as well as a floor. BS EN 12056-2 states a maximum gradient for branch pipework, and the reason is the one every drainage engineer repeats: on a steep short branch the water outruns what it is carrying and the solids are left on dry pipe. A branch that is over-pitched to clear a joist and then flattens out for the last two metres is worse than one laid consistently at the minimum, because it manufactures the exact velocity change that drops a load.
Run the drop for the worst route first — the longest one, the one with the awkward corner — and see whether the invert at the far end still clears the structure and lands above the connection point. If that one fits, everything shorter fits.
| Developed length | At 1/4 in per ft | At 1/8 in per ft |
|---|---|---|
| 3 m (about 10 ft) | 63 mm (2.5 in) | 31 mm (1.2 in) |
| 6 m (about 20 ft) | 125 mm (4.9 in) | 63 mm (2.5 in) |
| 9 m (about 30 ft) | 188 mm (7.4 in) | 94 mm (3.7 in) |
| 12 m (about 39 ft) | 250 mm (9.8 in) | 125 mm (4.9 in) |
Put the developed length of the real route in, not the straight-line distance across the room, and the minimum drop comes back as a height you can measure against the joists you are standing on.
The horizontal length of the drain pipe run.
Larger drain pipes are allowed a gentler minimum slope.
Minimum required drop
4.875 in of drop (minimum)
This is the code minimum slope — always check your specific local plumbing code, which may set stricter requirements in some jurisdictions.
- Pipe run length
- 19.5 linear ft
They open the calculator with your figures already in it
Drain Pipe Slope Calculator: 4.88 in of drop (minimum) — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 4.875 in of drop (minimum) — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- The size question is answered with two bands, so a 4 in branch and a 300 mm building sewer both land in the same band and get 1/8 in per ft. Real code tables are not that coarse: several add a gentler band for the largest sizes, and some jurisdictions require 1/4 in per ft at every size unless the reduction is specifically approved, so both the band boundary and the permitted figure are jurisdictional.
- Nothing here checks the drop you actually have against a maximum, and nothing checks that the fall will physically fit: a 20 m run of large pipe needs about 208 mm of drop at this slope, and that depth has to be available between the fixture connection and the invert of the sewer or septic tank at the far end.
- Slope is not capacity. Pipe size is set by the drainage fixture unit load on the run and the maximum permitted flow depth, and this page never asks how many fixtures discharge into it — the size band is something you tell the calculator, not something it works out for you.
- The length is treated as one straight horizontal run. No allowance is made for bends, offsets or developed length through fittings, and the page does not size or locate the vents, traps and cleanouts the drain also needs; a run at exactly the right slope will still siphon its traps if the venting is wrong.
- The figure is a total end-to-end drop and assumes the fall is delivered evenly along the run. It says nothing about hanger spacing or pipe bedding, and a single sag deep enough to hold standing water will collect solids even though the two ends of the pipe are the compliant distance apart.
Where the fall goes missing
The drop the calculation asks for sits on top of everything else that eats height, and three things eat it before you have finished. A swept ninety at the pan spigot consumes depth in its own right. Every change of direction taken as a bend rather than a slow sweep adds resistance paid for in gradient. And a branch that leaves the cupboard, crosses under a doorway and re-enters a floor void is often forced to its shallowest point exactly where the joists are deepest.
Structure decides more of this than hydraulics do. A hundred-millimetre pipe does not go through a solid timber joist at all: no notch of that depth is permitted and no bore of that diameter is legitimate in a solid member. Where the joists run across the route rather than along it, the honest options are a raised deck over the whole cupboard, dropping the branch into the ceiling void of the room below and accepting a boxed run, or moving the pan. Engineered joists change the picture but not the arithmetic — an I-joist has a web hole chart specific to its depth and series, and the manufacturer's evaluation report governs what may be cut, with the flange never in scope.
Then there is where the branch is allowed to arrive. Approved Document H keeps other connections clear of a zone below a WC entry on a stack so that a discharging WC cannot cross-flow into a neighbouring branch, and it also sets a minimum height for a ground-floor connection above the invert of the bend at the foot of the stack, increasing with the number of storeys above. Both rules routinely delete the connection point a designer had assumed was available, and they do it after the pan position is already on the drawing.
Buying the height back before buying a pump
A gravity route that misses by fifty millimetres is not a failed gravity route; it is an unresolved one. Every option below is cheaper over twenty years than a mechanical device, because none of them has a motor, a seal, an impeller or an alarm, and none of them needs a circuit that must be left energised while the house is empty.
Take them in the order that disturbs least, and concede on a measurement rather than on a feeling that the run looks tight.
- Re-route rather than re-level: walk the pipe the long way round the stair if that route crosses fewer joists, since a longer run at a legitimate gradient beats a short one that has to pass through structure.
- Raise the finished floor of the compartment on a platform, and check the resulting headroom under the stair string and the door swing before committing to it.
- Drop the branch into the ceiling void below and box it, agreeing the boxing with whoever owns that room before the pipe is cut.
- Move the connection instead of the pan: a new boss on the stack at a lower level, or a new junction into the underground drain outside the wall, can be worth more fall than anything done indoors.
- Re-plan the compartment so the pan sits on the wall nearest the stack, and give the basin the long waste run instead, since a basin waste is small, shallow and far easier to fall.
- Where the fixture sits below the level of the next upstream manhole cover, price a backwater valve into whichever route survives, because the adopted code will require protection against surcharge regardless of how the water gets there.
When the run genuinely does not exist
Three different machines get called the same thing in conversation, and they are not interchangeable. A macerator sits behind or beside the pan, shreds the discharge and pushes it up a small-bore pipe. A packaged lifting station collects wastewater in a sealed vessel and pumps it out through a larger main without shredding, relying on the pump passing solids whole. A site-built ejector pit is the same idea in a hole in the floor, with the pump submerged and the chamber formed in concrete or dropped in as a basin.
The distinction is written into the standards, and in Britain the distinction is what makes an installation legal or not. BS EN 12050-1 covers lifting plants for wastewater containing faecal matter; BS EN 12050-3 covers lifting plants for wastewater containing faecal matter for limited applications, which is the small-bore macerator family. Approved Document H permits a WC on a macerator and small-bore discharge only where the building also has access to a WC discharging directly to a gravity system, and the unit meets the relevant part of BS EN 12050 in the edition in force. That single condition decides most of these jobs: an extra loo under the stairs in a house with a bathroom on the stack satisfies it, while a basement flat whose only WC would be the pumped one does not, and needs a full lifting plant instead.
North America frames it through product standards rather than through a second WC. Macerating toilet systems and their components are covered by ASME A112.3.4 and CSA B45.9, and the sumps and ejectors provisions of the adopted plumbing code govern the pit version — a gas-tight, vented receptacle, a stated minimum discharge size for an ejector receiving water closet discharge, and a pump rated to pass solids of a stated size. Two inches is the figure most often quoted for both the discharge and the sphere the pump must pass, and it is worth confirming against the adopted edition rather than against a supplier's datasheet.
Grinder pumps sit off to the side of this. They shred to a slurry and can therefore discharge through a small main against real head, which makes them the usual answer where a property has to pump a long way to a sewer rather than to a stack in the same building. They are also the least forgiving of what gets flushed.
A pit that takes WC discharge, taken apart
A sealed ejector pit is a small assembly with a large consequence, and the parts are bought and sized separately. The chamber is a volume; the pump is a flow; the rising main is a diameter; the vent is a code item; and the lid is the only thing standing between the discharge and the room it is under.
Size the chamber from what arrives, not from what is available. A cistern delivers a discrete slug rather than a trickle: six litres into an eighteen-inch basin raises the level by only about thirty-seven millimetres, so the float differential you choose is really a decision about how many flushes the pit banks before the motor starts. Too tight a differential and the pump short-cycles on every flush, which is what kills float switches and windings. Too generous and you are storing sewage under a floor for longer than anyone wants to think about.
The chamber also has to stay where you put it. A moulded basin set into a wet excavation will float before it is filled, so anchoring or ballasting it is a step rather than a precaution. Where the chamber is formed in concrete, the pour has a hollow middle and the volume is the block less the void.
A sealed ejector pit, lid to bedding
- Vent to atmosphere — the pit is a pressure source when the pump starts, so it is vented to open air the way a stack is and never relieved with an admittance valve tucked into the cupboard
- Bolted gas-tight lid — gasketed and mechanically fixed rather than dropped in place, because this is the only barrier between foul air and a habitable floor above it
- Rising main, check valve and isolating valve — the check valve keeps the column from falling back into the chamber and the full-bore isolating valve above it lets that check valve be changed without emptying the pit first Sump Pump Discharge Pipe Sizing Calculator
- Submerged pump and float — rated to pass solids whole rather than to move clean water, and switched on a differential wide enough that the motor gets a real run each time it starts Sump Pump GPM Calculator
- Chamber shell — the banked volume between float levels is what decides cycle frequency, and a formed chamber is priced as a solid pour less the void it surrounds Sump Pit & Catch Basin Concrete Calculator
- Bedding and surround — compacted support under and around the chamber, plus the anchorage that stops a light basin floating out of a wet excavation before it is loaded Trench Excavation & Backfill Volume Calculator
Set the basin diameter and the depth between float levels, then ask how quickly a fixture group can fill that band — the flow that comes back is the floor the pump has to hold at working head, not at zero head.
The inside diameter of the sump basin/pit.
The depth of water the basin fills before the pump should activate and clear it.
How quickly the basin fills to the 'on' level during heavy inflow.
Minimum pump capacity, GPM
6.47 GPM (minimum)
This sizes for the basin's fill rate only — also check the pump's rated GPM at your actual vertical lift height (to the discharge point), since GPM drops as lift height increases on every pump's performance curve.
- Basin volume at fillable depth
- 12.94 gal
They open the calculator with your figures already in it
Sump Pump GPM Calculator: 6.47 GPM (minimum) — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 6.47 GPM (minimum) — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- The number is an inflow rate, not a flow a pump will deliver. Vertical lift, discharge-pipe friction, elbows and the check valve all cut into a pump's rated output, and only the lift is even mentioned on this page. Size from the pump's performance curve at your actual total head, never from its headline GPM.
- The basin is modelled as a bare cylinder. The pump body, the float, the check valve and any gravel in the pit bottom all displace water, so the real volume between the float levels is smaller than the volume shown — and an inflow rate worked back from a timed fill in a real pit is lower than what this returns.
- No margin is added. A pump rated at exactly this flow only breaks even with the inflow: it never gains on the water and never shuts off. The model also says nothing about cycle rate or how long the motor can run continuously.
- The fill time is an assumption you supply, not a derived quantity. Nothing here reads rainfall intensity, the footing-drain or roof area feeding the pit, or a rising water table, so the answer is only as good as the worst-case fill time you entered.
- This is not a drainage or waterproofing design and not a backup-power calculation. It does not cover where the discharge may legally terminate — that is set locally and sanitary-sewer discharge is restricted in many places — nor freeze protection on the discharge line, backflow prevention, or the capacity you need when mains power fails during the storm that caused the inflow.
The rising main has two limits, not one
Sizing a clean-water sump discharge is a one-sided problem: keep velocity below a practical ceiling so friction and water hammer stay manageable, and take the next standard size up. A main carrying faecal wastewater has a second limit underneath the first. If the velocity falls too low, solids drop out and build up on the invert of the rising main, and a main that silts is a main that has to be cut open. BS EN 12056-4 is where the minimum flow velocity for a pumping main comes from, and the figure usually worked to is around 0.7 m/s.
That turns pipe selection into a band rather than a floor, and the band is narrow on a domestic installation. Going up a size to reduce friction can drop the velocity under the scouring minimum at the flow the pump actually delivers. Size for the ceiling first, check the result against the floor at the operating flow, and reconsider the pump rather than the pipe if the two cannot both be met.
Small-bore macerator discharge is a separate problem governed by the unit's own instructions rather than by pipe hydraulics. Twenty-two and thirty-two millimetres are the usual sizes, the vertical section goes in immediately at the unit before any horizontal travel, and the horizontal leg then falls gently to the point of connection. Saniflo's installation instructions publish the trade-off between the two as roughly one metre of vertical lift against ten metres of horizontal run, and both Saniflo and Grundfos state a maximum for each in the literature for the specific model. Neither figure is transferable between units.
Whatever the size, resist the instinct to add a second check valve downstream of the one the manufacturer supplied. Two non-return valves in series on a short main trap a column of liquid between them that the pump cannot shift and the drain cannot drain, and the symptom is a unit that runs, sounds normal and moves nothing.
This sizes the main against the upper velocity limit, which is the right question for clean water and only half the question for sewage — take the diameter it gives, then work the velocity back out at your real flow and check it has not dropped below the scouring minimum.
The sump pump's rated discharge flow rate.
The maximum velocity allowed in the discharge pipe to limit friction loss and water hammer risk — commonly around 1.5 m/s (about 5 ft/s).
Minimum discharge pipe diameter
1.822 in
They open the calculator with your figures already in it
Sump Pump Discharge Pipe Sizing Calculator: 1.82 in — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 1.822 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- Returns an INSIDE diameter, and pipe is sold by nominal size. Wall thickness is the difference: a heavier schedule at the same nominal size has a smaller bore, so stepping up one nominal size can still leave you under the diameter calculated here. Check the published inside diameter for the material and schedule being bought, not the size printed on the pipe.
- Can return a diameter smaller than the pump's own discharge tapping, and that is not permission to reduce it. The port size is set for the impeller behind it, so necking a 1-1/2 in (38 mm) outlet into a 1-1/4 in (32 mm) line adds head the pump was never rated against and runs it off its curve. Treat the port size as the floor this result sits on top of.
Run time, cycle count and the life of the motor
A pump specification is two numbers that have to agree: it must move the banked volume faster than the fixtures can refill it, and it must run long enough each time to avoid short-cycling. Total dynamic head is the vertical lift from the pit to the point of discharge plus the friction through the main, the elbows and the check valve, and a unit advertised at a flow measured with an open outlet delivers a fraction of that once three metres of lift and fifteen metres of pipe sit in front of it.
The number worth knowing is the run time per cycle at that operating point, because it converts directly into starts per hour, and starts per hour is what motor manufacturers actually limit. Take the drawdown volume between float levels, put it against the flow the pump can hold at the real head, and see how long a cycle lasts. If the answer is eight seconds, the differential is too tight regardless of what the pump is rated at.
Treat any modelled figure as an approximation and let the manufacturer's published curve govern the purchase. A smooth-bore friction model and a straight-line pump curve are the standard first approximations for small submersibles, close enough to tell you whether a proposal is sensible, wrong by a factor, or marginal.
Enter the volume banked between the float levels rather than the whole pit, with the real lift and the real length of main, and the time it returns is one cycle — divide the hour by it to see how often the motor is being asked to start.
The volume of water to move — a pool, a flooded floor, a tank.
The headline flow on the pump's box or plate — measured with no lift and no hose.
The lift at which the pump's flow falls to zero — on the same plate as the rated flow.
Height from the water surface up to the hose outlet.
The inside bore of the discharge hose — the single most sensitive number here.
Total discharge hose length, including the part lying flat.
Estimated drain time
10.4 hours
Built on a linearised pump curve and Hazen–Williams hose friction (C = 150). A manufacturer's published curve, where you have one, governs.
- Delivered flow at the operating point
- 21.14 gal/min
- Share of rated flow surviving lift and hose
- 39.88 %
- Friction head in the hose
- 9.13 ft
- Total dynamic head
- 15.63 ft
They open the calculator with your figures already in it
Pump Drain Time Calculator: 10.42 hours — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
What this calculation does not cover
- The vertical lift is held constant for the whole job and no water is assumed to arrive while the pump runs. In a pool the lift grows as the level drops; in a flooded basement groundwater, rain or a live leak keeps feeding the space. Real time to empty runs longer than this figure, and where inflow matches the delivered flow the level never falls at all.
- Only straight hose length is charged for friction. Fittings, quick-couplers, a strainer or foot valve, a check valve, kinks, a partly closed discharge and any suction-side pipework all add head that is not in this number. Hazen-Williams with C = 150 is an empirical fit for smooth pipe carrying clean water — silt, debris, cold water or a ribbed corrugated hose all move the real loss away from it, and the formula is at its weakest in bores this small.
- The pump is modelled as a straight line between the two figures on its plate. Where you have the manufacturer's published curve, that curve governs. Nothing here checks that the pump can prime at your suction lift, that it will hold prime as the water shallows, or that it is rated to run continuously for the hours reported — most utility pumps also stop drawing well before the floor is dry.
- This is a time estimate, not a decision to empty the pool. It says nothing about hydrostatic uplift on an empty shell, liner shrinkage or collapse, or the groundwater conditions that decide whether a pool can safely be drained at all. Those questions belong to the pool builder and to the shell's own relief arrangements.
- Where the water goes is outside this calculation. Discharging chlorinated, salt or silt-laden water to a storm drain, a watercourse or adjoining land is regulated in most places, and the receiving drain's capacity may be far below the flow reported here. Confirm the disposal route with the local water authority before running the pump.
What comes back down the pipe when the pump stops
Every rising main holds a standing column, and the moment the pump switches off that column tries to return. A thirty-two millimetre bore holds about eight tenths of a litre per metre; a fifty millimetre bore holds close to two litres per metre. On a three-metre lift in fifty millimetre pipe that is nearly six litres — a whole flush — falling back into the chamber at the end of every cycle if the check valve is at the top of the riser or has failed.
The consequence is not only a wasted cycle. Fall-back re-fills the pit immediately after it has been emptied, which drags the float back toward the start level and produces the double-start that reads on site like an inflow problem. Work out the column volume before you position the valve, keep the check valve low and close to the pump, and where the main runs somewhere that freezes, know how much liquid is standing in it overnight rather than guessing.
Use the bore rather than the nominal size and the vertical height rather than the total run, and the answer is the volume the check valve is holding up — the same volume that returns to the chamber the moment it does not.
The pipe's inside diameter, not the nominal or outside size.
The total length of pipe run.
Estimated pipe volume needed
0.7573 gallons
- Volume (liters)
- 2.87 liters
- Volume (cubic in)
- 174.95 cubic in
They open the calculator with your figures already in it
Pipe Volume Calculator: 0.7573 gallons — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- The figure is the contents of a straight bore and nothing else. Fittings, valve bodies, meters, strainers and any water standing in a cylinder or tank at the end of the run are not counted, and the pipe wall is excluded — so this is not the volume of pipe material, and not the space the pipe occupies in a chase or trench.
- Volume scales with the square of the bore, so an inside diameter that is 9 per cent out produces a volume 19 per cent out. Published inside diameters differ by material, schedule and class for the same nominal size, and the calculator takes whatever figure you type at face value — it has no way of knowing that a half-inch pipe was entered at half an inch.
- It assumes the run is completely full of liquid over its whole length. Gravity drains, waste stacks and sewers are designed to flow part full, and a system that has not been purged holds air at its high points, so both contain less than this number says.
- It answers how much the run holds, not how long the wait is. Turning that volume into a hot-water delay needs the fixture's flow rate as well, and the real delay runs longer because the first hot water gives up heat to the pipe wall and to whatever surrounds it. No time calculation happens on this page.
- Nothing here is a sizing or a support check. Velocity, friction loss and pressure drop are separate calculations, and a long run that holds a comfortable volume can still be too small to deliver flow. The contents also weigh whatever that volume of water weighs, which bears on hanger spacing and is not assessed.
The supply the new room also needs
A pumped WC still needs a cistern, and a cistern still needs a supply, and the spur to a cupboard under the stairs is often teed off whatever pipe happens to pass nearby. The load itself is trivial — a WC and a small basin add only a couple of water supply fixture units, read off the adopted code's table — but the branch it is teed from was sized for the fixtures that already existed, and the relevant question is what the addition does to that branch rather than what the two new fixtures demand on their own.
The fixture-unit method works because fixtures are used intermittently and rarely together, and that diversity is why the pipe is far smaller than the sum of the flow ratings suggests. At two to four fixture units, though, a square-root approximation of the demand curve is being asked to work at the very bottom of its range, where it is least reliable. Treat the diameter it returns as a floor to check against, and let the adopted code's own table settle the size.
The practical failure here is rarely capacity anyway. It is a long dead leg to a single float valve, a cistern that refills slowly whenever a shower runs upstairs, and an isolating valve that will be behind a tiled panel by Friday. Put the isolator where a hand reaches it, and give the macerator unit its own accessible position for the same reason.
Load the branch you are teeing into with the fixtures already on it plus the two you are adding, and read the result as a minimum to sanity-check the code table against rather than as the size to buy.
The sum of water supply fixture unit values for every fixture served by this pipe segment.
A calibration constant that scales the square-root approximation to your building's fixture mix.
The maximum velocity allowed in the pipe to limit noise and water hammer risk.
The pressure available where the supply enters the building.
The length of pipe from the point of supply to the furthest fixture, following the run.
How much extra length is added to represent the elbows, tees and valves in the run.
The height of the highest fixture above the point of supply.
The pressure the metering assembly loses at your design flow.
The flow pressure the furthest fixture needs at its inlet to work properly.
Which Hazen-Williams roughness coefficient the friction calculation uses.
Minimum pipe diameter
0.862 in
Velocity governs this run. The diversified demand would fit through a smaller bore on pressure alone, but sustained velocity above the ceiling erodes the pipe and is audible through the building, so the velocity limit is the binding one and the pressure budget has room to spare.
- Estimated peak demand
- 14.31 GPM
- Diameter set by the velocity ceiling
- 0.86 in
- Diameter set by the pressure budget
- 0.77 in
- Pressure lost to static lift
- 58,286.8 Pa
- Pressure lost to friction at this diameter
- 122,120.36 Pa
- Pressure left at the furthest fixture
- 198,803.83 Pa
- Equivalent length used for friction
- 147 ft
They open the calculator with your figures already in it
Domestic Water Pipe Sizing by Fixture Unit Calculator: 0.8616 in — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 0.862 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- Fixture-unit methods work because fixtures are used intermittently and rarely together. That diversity is the whole basis of the sizing, and it is why the pipe is far smaller than the sum of the fixture flows would suggest.
- Hunter's curve is old, and its age biases this in one direction. It was derived in the 1940s from fixtures using several times the water modern ones do, and it is well documented as oversizing systems built with low-flow fittings — the demand factor is the handle for that, and lowering it is a judgement with consequences rather than a correction.
- Excludes continuous-demand loads such as irrigation and hose bibs, which do not benefit from diversity and are added at their full flow.
- The friction calculation assumes one diameter for the whole run. A real system steps down as branches leave it, so the true loss is somewhere between this figure and the loss of a system sized entirely at the smallest branch — this is the screening answer, and a segment-by-segment calculation is the design one.
- Fitting losses are an allowance, not a count. Each elbow, tee and valve has its own equivalent length and a push-fit insert reduces the bore as well, so the percentage is a stand-in for a takeoff nobody has done. On a run with many tight bends it will understate the loss.
- The meter figure is yours to supply and this page cannot check it. Meter loss rises steeply as flow approaches the meter's rating and a backflow preventer or pressure-reducing valve often costs more than the meter itself; entering a nominal figure where the real assembly loses far more is the quiet way this calculation goes wrong.
- Velocity limits govern independently of flow: sustained velocities above roughly 2.4 m/s (8 ft/s) cause erosion and noise regardless of what the fixture units allow.
- The result is an internal diameter, not a pipe size. Nominal designations are not bores — copper, PEX and CPVC of the same nominal size have materially different internal diameters, and a push-fit system loses more at every joint — so take the next size up whose actual bore meets this figure in the material you are using.
Air, power and the alarm nobody fits
Ventilation is a code requirement for the compartment as well as for the drainage. Approved Document F sets an extract rate for sanitary accommodation and the International Residential Code's light and ventilation provisions do the equivalent through either an openable area or mechanical extract at the rate in the mechanical chapter's table. Under a stair there is usually no window, so it is a fan, ducted to outside — terminating an extract in a floor void or a loft moves the moisture rather than removing it.
The electrical side is where a pumped WC differs most from a gravity one, because it introduces a permanently energised appliance into a room full of water. In the United Kingdom, BS 7671 Section 701 only bites where the compartment actually contains a bath or a shower — a cloakroom with a pan and a basin is an ordinary location, and the special-location zones people quote at it do not apply. Where a shower does go in, Section 701 dictates where a supply may be taken, what protection it carries and what may be installed in which zone, and a macerator is then normally fed from a fused connection unit sited outside the room. In North America, NFPA 70 Article 210.8 requires ground-fault circuit-interrupter protection for the receptacles involved, and an ejector pump is normally given its own circuit so that nothing else can trip it.
Then fit the alarm, the item most often deleted on price and the only thing that turns a pump failure into an inconvenience instead of a flood. A high-level float on a separate supply, sounding somewhere a person actually is rather than in the cupboard it monitors, buys the hours needed to stop using the fixture and get someone out. Where the household has no other WC, the same argument leads to duty and standby pumps on staggered floats — an easier conversation before the floor is closed than after.
Pricing two answers that are not the same shape
The fit-out of the room is the easy half and is broadly the same whichever route wins: a pan, a basin, a small tiled and tanked area, an extract fan, a light and some decorating. Price that on its own and keep the drainage route out of it — a fit-out figure that has quietly absorbed a soil connection is how a modest downstairs loo becomes a structural project without anybody noticing where it happened.
Then price the route as its own line, twice. The gravity line carries the branch pipe, the fittings, the stack boss or the new junction outside, the structural work needed to get through or around the floor, and the making good. The pumped line carries the unit, the rising main, the connection at the far end, the dedicated circuit, the alarm, the chamber and its excavation where there is one — and a service interval that repeats every year for as long as the fixture exists.
Set the contingency honestly on the gravity option in an older property, because lifting a floor is how you discover that somebody has already notched the joists for a waste run that was never signed off. The pumped option carries less of that risk and more of the other kind: it is the option whose cost keeps arriving after handover.
This prices the room and deliberately excludes moving the soil connection, which makes it the right tool here — run the fit-out once, then add the gravity route and the pumped route beneath it as separate figures so the comparison is between the two things that actually differ.
Bath, shower, basin, WC, bidet — each counts as one.
Item, pipework and fitting labour per fixture.
Walls plus floor.
Tile, adhesive, grout and labour.
Wet zone requiring a membrane.
Membrane or liquid system, applied.
Fan, ducting and its electrical supply.
Lighting, shaver point, heated rail, paint.
Higher where the existing floor is unknown.
Total bathroom cost
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Bathroom Remodel Cost Calculator — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
What this calculation does not cover
- Excludes moving the soil stack or the WC to a materially different position, which changes the drainage design rather than the fit-out.
- Excludes structural repair to a floor found to be inadequate once lifted — a common discovery and the main reason the contingency here is set higher than for other rooms.
Handover, and the label on the lid
Commissioning a pumped WC means loading it the way the household will. Flush it repeatedly rather than once, watch a full cycle from start to stop, and listen for the check valve seating cleanly instead of slamming. Then leave the owner something written down: the next person to look at this will be doing it in the dark with water on the floor.
A gravity branch is handed over differently and more quietly: photograph the run before the floor closes, mark the joist positions and the invert at each end on the as-built, and note where the rodding access is. In year eight, that photograph is the difference between lifting two boards and lifting a room.
- Flush four times in succession and confirm the chamber clears fully and the pump stops rather than hunting on the float.
- Check the check valve seats without hammer, and if it slams, deal with it as a surge problem rather than by tightening something.
- Isolate the supply to the unit at the panel and confirm the high-level alarm sounds where it can be heard from a living space.
- Prove the isolating valve above the check valve closes fully, so the check valve can be changed later without the standing column in the main draining back into the chamber.
- Confirm the lid gasket is seated and every fixing is home, then check for odour after a full day of use.
- Leave the model number, the discharge size, the vent route and the service interval written on a label fixed inside the compartment, not on a sheet of paper in a drawer.
Settle before the pan position is fixed
Take these off the emptied cupboard and the exposed floor, in this order. The first three decide whether there is a gravity route at all; the rest only matter once that answer is no.
- Developed length of the real route — Followed round the stair and along the walls, not measured straight across the room, because the pipe cannot go that way.
- Fall available between pan outlet and connection invert — Measured to the connection you are actually permitted to make, allowing for the crossflow zone and the stack-base height the code protects.
- Joist direction, depth and species along the route — A hundred-millimetre pipe does not cross a solid joist; for engineered members the manufacturer's web hole chart governs and the flange is never in scope.
- Which BS EN 12050 part or product standard the unit is listed to — The limited-application family carries the condition that another WC discharges directly to gravity; a sole WC needs a full lifting plant.
- Static lift, main length and fitting count on the discharge — Total dynamic head, not lift alone, and the flow the pump holds at that head rather than the number printed on the carton.
- Ventilation, dedicated circuit and alarm position — Extract to outside at the code rate, GFCI or RCD protection to NFPA 70 Article 210.8 or BS 7671 — Section 701 only if a shower goes in — and an alarm audible from a living space.
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