Turnover is the specification; everything else is arithmetic
Turnover is the time a circulation system needs to pass a volume of water equal to the pool's contents through the filter and return it to the tank. It is a design period, not a measure of cleanliness: no pool ever exchanges its water in neat, discrete batches, because returned water immediately mixes with water that has already been treated. A four-hour turnover does not mean every litre has been filtered after four hours. It means the plant is moving enough water that the dilution curve stays ahead of the contamination the bathers bring in.
Fix that one period and a chain of consequences closes behind it. Design flow rate falls out of volume divided by turnover. Filter area follows from flow rate and the permitted filtration velocity. Pipe diameters follow from flow rate and velocity limits. Pump duty follows from flow rate and the head the pipework generates. Heater output, chemical feed rate, sample line flow, electrical load and annual running cost all sit downstream of the same figure.
Error in either direction is expensive and visible. Too slow, and the symptoms arrive as cloudy water on a busy Saturday, combined chlorine that will not clear, failed microbiological samples and a filter that never sees enough flow to backwash properly. Too fast, and the plant burns energy continuously, the filter runs above its rated velocity so fines pass straight through it, and the pool spends its life fighting a duty it never needed.
An honest volume, before anything gets sized
Nothing downstream survives a wrong volume. A ten per cent error carries straight into turnover flow, filter selection, every chlorine dose and every acid correction the pool will ever receive, and it compounds for the life of the installation. Volume taken off a sales brochure, a tile plaque, or the previous installer's word is not a measurement — plenty of pools have been built to a drawing and then finished with a shallower hopper, a wider Roman end, or a tiled bed that stole depth all along.
Measure the shell as it was actually built. Take the water line as your datum rather than coping level, since the operating level sits below the tile band. Break irregular shapes into prisms you can defend and use the true average of shallow and deep ends across the transition slope rather than an eyeballed mid-depth. Steps, benches and swim-out shelves come off the total; on a small pool they are not a rounding error.
Where a deck-level pool runs to a balance tank, state which volume any given figure refers to. Turnover is normally set against the pool contents, while the tank carries surge and top-up. Quoting a combined number on one document and a pool-only number on another is a reliable way to produce a plant that sizes correctly on paper and dawdles in service.
Every figure in the rest of this guide is a multiple of the water volume, so it belongs at the very front of the sequence rather than as a check at the end.
The outline of the water surface seen from above.
The length of the pool.
The width of the pool.
Work the volume at one depth, or at the average of a shallow end and a deep end.
The water depth, or the average depth if you already know it.
Estimated pool volume
20,370 gallons
A rectangle's water surface is its length times its width. The depth entered is applied across the whole of that area. The volume is that area times the depth.
- Volume in cubic feet
- 2,722.5 ft³
- Water surface area
- 544.5 sq ft
- Depth the volume is worked at
- 5 ft
They open the calculator with your figures already in it
Pool Volume Calculator (Rectangular, Round, Oval or Kidney): 20,366 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
- One depth is applied across the whole plan, whether it is typed in or worked from the two ends. Averaging the shallow-end and deep-end figures is exact only where the floor falls at one steady rate from end wall to end wall, or where the shallow and deep sections cover equal plan area. On the usual arrangement - a large flat shallow section, a slope, then a smaller deep end - the plain average overstates the volume, and everything built on that figure inherits the overstatement in the same direction.
- Nothing inside the outline is deducted. Steps, benches, tanning ledges, swim-outs, beach entries and coved or radiused corners all occupy space this calculation fills with water. A plan that is none of the five shapes here - an L, a lazy L, a Roman or Grecian end, a true freeform - is outside the model; split it into pieces the page does take, work each at its own depth and add them.
- Only the water standing in the pool basin is counted. Pipework, skimmers, the filter, the heater and any attached spa, balance tank or vanishing-edge catch basin hold further water that circulates through the system and has to be dosed with it, and none of that appears in this figure.
- This is a capacity estimate, not a dosing instruction. How much of any product to add depends on its active strength, your current test readings and your target concentration, and an overstated volume overdoses every time it is used. Dose once, circulate a full turnover, then test before dosing again - never dose twice off the same volume figure.
- The fill itself is not covered: no source flow rate, no fill time, no metered water cost, and no account of the hardness, alkalinity or metal content of the incoming water, which is what sets the pool's starting chemistry and how much correction it needs on day one.
Converting the period into a duty the plant must hold
Turnover only becomes a specification once it is expressed as flow — cubic metres per hour or litres per second that the circulation has to sustain. That conversion is where domestic and commercial work part company, because the periods differ by an order of magnitude and the resulting pipe and filter sizes differ with them.
Sustain is the operative word. The flow figure has to be met at the dirty end of the filter cycle, when differential pressure has risen and the system curve has steepened, not on the day of commissioning with clean media and a fresh strainer basket. Specify the duty at the backwash trigger condition and the pool will hold its turnover throughout the cycle; specify it clean and it will drift below design for most of every week.
On multi-pump installations, decide early whether the pumps are duty-and-standby or duty-and-assist, and whether design flow must be met with one machine down. That decision changes pipe sizing, header arrangement and the filter count, and retro-fitting the answer after the plantroom is poured is rarely cheap.
At this point the volume needs to become litres per second before pipe, filter or pump can be chosen, and that is precisely the conversion this calculator performs.
Water volume.
Complete circulations of the whole volume each day.
Hours the pump actually runs.
Flow rate required
27.52 gal/min
Flow required at the pump. The pump must deliver this against the system's actual head, which is a separate calculation.
- Flow rate in m³/hr (metric filter and pump rating basis)
- 6.25 m³/hr
- Volume circulated per day
- 65.4 yd³
- Turnover time
- 8 hours
They open the calculator with your figures already in it
Pool Pump Turnover Calculator: 27.52 gal/min — 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 — 27.52 gal/min — 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
- Says nothing about head. A pump rated at this flow on an open discharge will deliver less through a filter, heater and long pipe run.
- Filter and pipe sizing cap the flow that can be used. Pushing more through an undersized filter shortens its life and reduces its effectiveness.
The pipework votes on whether that flow is deliverable
Design flow is a statement of intent until the pipework can carry it. Velocity governs: suction lines are held to a lower limit than returns because suction-side losses eat directly into net positive suction head, and a pump starved on the suction side cavitates, loses flow and destroys its own impeller. Return-side velocity has more latitude but not unlimited — noise, water hammer on valve closure and friction loss all climb steeply with it.
Undersized suction is the most common defect found on existing plant. It shows as a pump that sounds like it is passing gravel, a strainer lid that collapses inward, air trapped at the pump crown and a flow meter reading well under design that no amount of impeller change will fix. The remedy is pipe, not pump, and it usually means opening the plantroom floor.
Outlet arrangements carry a safety dimension that has nothing to do with turnover but is constrained by it. Suction outlets must be sized, split and covered so that no single blockable outlet can develop entrapment force, which caps the flow any one drain may draw. Requirements are set by product and installation standards — EN 13451-3 for pool inlets and outlets in Europe, ANSI/APSP-7 on suction entrapment avoidance and the Virginia Graeme Baker Pool and Spa Safety Act in the United States — and they will sometimes force a second outlet or a larger sump on you before the hydraulics do.
| Shorten the turnover | Immediate consequence | What usually becomes the limit first |
|---|---|---|
| Design flow rises | Higher filtration velocity | Filter bed area or filter rated flow |
| Design flow rises | Higher pipe velocity | Suction pipe diameter and NPSH available |
| Design flow rises | Higher total head and absorbed power | Pump motor size and electrical supply |
| Design flow rises | Larger backwash demand | Waste line capacity and dump/soakaway |
| Lengthen the turnover | Lower energy use per day | Skimming performance and heater minimum flow |
Velocity is what a pipe size means in practice: past about 6 ft/s suction lines cavitate and returns whistle, and no pump selection recovers a bore chosen too small.
The design flow rate through the backflow preventer.
The internal diameter of the pipe at the backflow preventer connection.
Flow velocity
484.1 ft/min
They open the calculator with your figures already in it
Backflow Preventer Discharge Velocity Calculator: 484 ft/min — 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 — 484.1 ft/min — 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 relief port is the flow that floods plant rooms, and it is not this one. A reduced-pressure assembly is built to dump water to atmosphere when a check fouls or downstream pressure rises, and the rate at full relief can be a large fraction of line flow — far more than a routine floor drain and air-gap fitting will swallow. That receptor is sized from the manufacturer's published relief discharge for the specific model and size, and it is what decides whether a failure is a puddle or a ceiling coming down on the floor below.
- The velocity limit that bites first usually belongs to the pipe, not the assembly. Copper thins by erosion-corrosion above roughly 8 ft/s (2.4 m/s) cold and about half that on hot recirculating lines, and it thins at the elbow and tee immediately downstream where the flow is still disturbed by the device — with plumbing codes capping velocity for noise on top of that. A figure comfortably inside the assembly's rating can still be outside what the pipe carrying it will survive.
Duty point, not the number printed on the pump
A pump has no single flow rate. It delivers wherever its curve crosses the system curve, and the system curve is static lift plus friction plus filter differential — a moving quantity that climbs through every filter cycle. The catalogue flow figure describes a test rig, not the pool in front of you.
Total head has to be built up honestly from the actual run: fittings, valves, heater, ultraviolet or ozone chamber, the filter at its dirty-condition differential, and the height from the water surface to the highest point of the return. Chambers and heat exchangers are the losses most often left out, and they are frequently the largest single contributors on a small plantroom.
An oversized single-speed pump does not deliver a generous safety margin. It runs off to the right of its curve, drags the filter above its rated velocity, pushes velocity in the suction line past sensible limits and delivers far less useful gain than the power bill suggests. Matching the duty point is more valuable than buying headroom.
The filter sets the ceiling on how fast you may go
Filtration velocity — flow per unit of bed area — is the constraint that turns a desirable turnover into an achievable one. Push a sand bed above its permitted velocity and it stops behaving as a depth filter: fines channel through, the water goes flat and grey, and no increase in coagulant recovers it. Media filters, cartridge units and pre-coat units all carry a manufacturer's rated flow, and permitted velocities for commercial installations are materially lower than the figures commonly accepted on private pools.
Backwash, not filtration, often decides the plant layout. Lifting and fluidising a bed demands a considerably higher flow than filtering it, which drives the waste line size, the discharge route and sometimes the pump selection itself. A plant that can filter beautifully but cannot lift its own bed will silt up and stay that way.
Media depth, freeboard above the bed and correct lateral distribution matter as much as area. Beds left short of depth after a media change, or filled so high that there is no room for expansion, will pass debris and lose media to the pool. Record the media type, depth and rated flow on the plant schedule so the next engineer is not guessing.
Turnover only counts if the water actually moves
A plant meeting design flow can still leave a pool with stagnant corners. Circulation is a distribution problem as much as a volumetric one: inlets have to be positioned and directionally set so that returned water sweeps the far corners and the floor rather than short-circuiting straight back to the skimmers. Steps, swim-outs, corners behind ladders and the underside of moveable floors are the usual dead spots.
Surface removal carries most of the load, because oils, sunscreen, organic debris and the bulk of bather contamination collect in the top few centimetres. Skimmer draw has to be balanced against floor outlets, and a deck-level system's channel and balance tank sizing determines whether the surface layer is genuinely being carried away or merely rocking back and forth.
Dye testing is the only honest proof. Introduce dye at the extremities and time its arrival at the outlets; the result exposes short-circuiting that flow readings will never show. Do it at commissioning, record the outcome, and repeat it whenever inlets are altered or a moveable floor is installed.
Chemistry inherits the turnover figure
Disinfection is a contact problem, and circulation is what creates the contact. Chlorine demand is generated in the pool by bathers and destroyed both in the pool and, where dosing is on the return, in the plant; the rate at which treated water reaches the far end of the tank is a function of turnover and distribution. Slow the circulation and residual becomes unevenly distributed long before the average reading moves.
Dose calculations are volume-based, so they inherit the volume error discussed earlier along with the turnover assumption. A dose that assumes instant mixing will read low at the sample point for a period governed by how quickly the plant can circulate the tank — which is a reason for controllers to be tuned against the actual turnover rather than a default.
Combined chlorine and total dissolved solids are managed by dilution, not by circulation, and that distinction is worth holding firmly. Fresh water replacement, expressed per bather, is a separate obligation from turnover; a plant with a short turnover and no dilution regime will still accumulate chloramines and by-products. Balance itself — pH, alkalinity, calcium hardness and temperature, assessed together through the Langelier Saturation Index — decides whether that circulating water protects the plaster, grout and heat exchanger or quietly eats them.
Interlocks belong on the sample line and the dosing circuit. Where flow fails, dosing must stop; a controller that keeps injecting into a stalled line will produce a slug of concentrated chemical at the next start and can damage the heat exchanger or the pool finish.
Dose is set against the same volume the turnover was, so a volume estimated generously once is over-dosed every time thereafter.
Water volume of the pool.
Measured, not assumed.
Where you want it.
What you are dosing with.
Product required
0.165 gal
The dose arithmetic is reliable; the pool volume usually is not. Add, circulate, then retest rather than dosing twice from a calculation.
- Rise required
- 1.5 ppm
- Pure available chlorine
- 75.01 g
- Product volume
- 0.17 gal
- Pool volume
- 13,210 gal
- Pool volume in US gallons
- 13,210 gal
They open the calculator with your figures already in it
Pool Chlorine Dose Calculator: 0.1651 gal — 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
- Liquid strengths are read the way the pool trade quotes them — available chlorine by VOLUME, so 12% means 120 g per litre. If your container states a WEIGHT percentage instead, divide this figure by the product's density; a 12% sodium hypochlorite solution is roughly 1.2 kg per litre (10 lb per US gallon), so the dose would be about a fifth smaller. Granular strengths are weight fractions either way.
- Assumes cyanuric acid is within range. Above roughly 100 ppm, chlorine is progressively locked up and the same dose achieves far less sanitiser activity.
- Liquid chlorine loses strength in storage — an old container may be well below its label percentage.
- Says nothing about pH, which governs how much of the free chlorine is in its active form. Chlorine dosed into high pH water does much less than the reading suggests.
The energy consequence of the number you chose
Circulation is typically the largest continuous electrical load on a pool, and its cost scales brutally with duty. Under the affinity laws, flow rises in proportion to pump speed, head with the square of speed, and absorbed power with the cube. Trimming speed by a fifth cuts flow by a fifth and power by roughly half, which is why variable-speed drives changed the economics of pool plant so completely.
Running longer at lower speed to achieve the same number of turnovers per day almost always costs less energy than running briefly at full speed, and it improves skimming and chemical distribution because the plant is on more of the time. Several energy standards now push installations in that direction — ANSI/APSP/ICC-15 on residential pool and spa energy efficiency being the widely referenced example — and some jurisdictions mandate variable-speed capability outright.
Floors exist under that logic, however. Heaters carry a minimum flow rate below which they will fault or scale the exchanger, ultraviolet and ozone units have a contact-time window, salt chlorinators and sensor cells need flow across the electrode, and automatic cleaners driven from the circulation need pressure. Set the low-speed duty above the highest of those minima, prove it at commissioning, and lock the settings so a well-meaning operator cannot dial the pool into a fault condition chasing the meter.
Heating is sized from surface area and the lift you want, and it is the load that turns a turnover decision into a running cost.
Water volume.
How far the water must be raised.
How quickly you want to reach temperature.
A cover suppresses evaporation, which is most of the loss.
Heater output required
23 kW
A planning figure. Standing loss depends on wind, humidity, air temperature and shelter, none of which are inputs here — it varies by a factor of two or more between a sheltered and an exposed site.
- Energy to raise the water
- 465.16 kWh
- Heat-up power
- 19.38 kW
- Standing loss allowance
- 3.33 kW
- BTU/h
- 77,508.05 BTU/h
- Approximate surface area used
- 358.83 sq ft
They open the calculator with your figures already in it
Pool Heater Sizing Calculator: 22.72 kW — 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 — 23 kW — 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
- Surface area is inferred from volume at an assumed 1.5 m (5 ft) average depth. A wide shallow pool loses far more heat than this suggests, and a deep narrow one less.
- Wind is the largest uncontrolled variable in evaporation and is not modelled. An exposed pool can lose twice what a sheltered one does.
- Solar gain is ignored, and on a sunny day it can exceed the heater's output entirely.
Proving the turnover exists, not asserting it
A flow meter on the return is the single most useful instrument in a plantroom and the one most often omitted. Install it with the manufacturer's straight lengths upstream and downstream — a meter mounted immediately after an elbow reads whatever it feels like — and site it where an operator can read it without a torch and a ladder.
Record two flows at handover: clean-filter and at the backwash trigger differential. Those two numbers, alongside pressure gauges before and after the filter, give the operator a complete picture of the cycle and tell any future engineer whether the plant has degraded or the filter simply needs backwashing. Without them, every subsequent fault diagnosis starts from zero.
Handover documentation should carry the volume, the design turnover, the design flow, the filtration velocity, media type and depth, pump model and duty point, and the dye test result. That page is worth more to the pool over twenty years than any other document in the file, and it takes an afternoon to produce properly.
Who actually sets the number
Turnover is not a matter of preference on commercial work. Public and semi-public pools are governed by the health authority or regulator having jurisdiction, and the required period varies with pool type: pools with high bather density relative to volume, shallow water and young users are always specified far shorter than deep main tanks, because contamination per litre is what the figure is really protecting against.
Reference documents differ by territory. In Europe, the EN 16713 series addresses water systems for private pools across filtration, circulation and treatment. In the United Kingdom, the Pool Water Treatment Advisory Group's code of practice for swimming pool water is the recognised commercial guidance, alongside the Health and Safety Executive's HSG179 on health and safety in swimming pools. In the United States, ANSI/APSP/ICC-11 addresses water quality in public pools and spas, while the CDC's Model Aquatic Health Code is a model document adopted, amended or ignored state by state and county by county.
Confirm the governing document before design, in writing, and record which edition applies. A pool designed to a general figure and inspected against a stricter local requirement is a rebuild of the plantroom, not a paperwork correction — and on domestic work, where the owner sets the period, the same discipline of writing the chosen figure down is what lets the next engineer service the pool rather than re-derive it.
Prove it before you leave site
Five records and checks that turn a claimed turnover into a demonstrated one.
- Measured pool volume on the plant schedule — Taken at operating water line from the shell as built, with steps and benches deducted; balance tank stated separately.
- Flow meter on the return with manufacturer's straight lengths — Log clean-filter flow and flow at the backwash trigger; both belong in the handover file.
- Pressure gauges either side of the filter — Sets the backwash differential and gives the operator the only cheap diagnostic they have.
- Filter media type, depth and rated flow recorded — Check freeboard for bed expansion and confirm the waste line can carry backwash, not just filtration, flow.
- Suction and return velocity check against design flow — Suction-side undersizing shows as cavitation and air at the pump crown; the fix is pipe, not a bigger impeller.
- Dye test at the pool extremities — The only proof that inlets are sweeping corners and steps rather than short-circuiting to the skimmers.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.