One measured number sets every other number
A drain field has no independent dimensions. Total absorption area, trench length, the number of laterals, the width of the reserve, the depth of stone under the pipe — all of them fall out of a single field measurement of how fast water leaves the soil at trench invert. Miss that measurement by a factor of two and the field is wrong by a factor of two, and no amount of careful pipe-laying recovers it afterwards.
Percolation rate is reported as minutes per inch of drop, so slow soils return large numbers and fast soils return small ones. Both ends are disqualifying. A heavy plastic clay can read so slowly that the health authority refuses a conventional gravity field outright and pushes the job toward a mound, an at-grade, or a pressure-dosed alternative. Coarse sand and gravel fail in the opposite direction: effluent moves through faster than the soil can treat it, and the design pivots to a sand-lined bed or an engineered treatment unit before it ever reaches native ground.
Sequence the work in one direction and never the other. Measure, describe the soil, derive the required area, then test whether that area fits the lot with setbacks honoured. Crews who begin by pacing out where the trenches would sit neatly and then hunt for a perc number that justifies the layout produce systems that pass inspection and surface in year six, when the excavator is long gone and the homeowner is the one calling.
Taking a rate the inspector cannot argue with
Presoak before you read anything. A hole tested dry reads fast because the soil is drinking rather than percolating, and every derived dimension downstream shrinks with it. Health authorities set their own presoak duration, usually extended in fine-textured soils, and that requirement lives in state or county sanitary code rather than in any national consensus standard. Confirm the local protocol before the auger leaves the truck — a set of holes run to the wrong procedure is a wasted mobilisation and a repeat trip.
Hole preparation carries as much weight as timing. Augering into wet soil glazes the sidewall, and a smeared bore reads several times slower than the same material scratched open with a knife or a length of rebar. Scarify the sides, clean the loose cuttings out of the bottom, and bed a couple of inches of coarse gravel so the refill water does not scour a crater and re-seal the base. Cut every hole to the depth the trench bottom will actually sit at, because horizons change over a few inches and a rate measured above the invert describes soil the effluent will never touch.
Spread holes across the whole footprint including the reserve area, and expect disagreement. A set returning 18, 22 and 61 minutes per inch is reporting a lens or a horizon boundary, and the correct response is more holes rather than an average that describes nowhere on the lot. Which hole governs — slowest, mean of qualifying holes, or a filtered set backed by a soil log — is a jurisdictional decision, so ask rather than assume. Back the numbers with a written soil description under ASTM D2488 Standard Practice for Description and Identification of Soils (Visual-Manual Procedures) and a classification under ASTM D2487 Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System), and dig a separate deep observation pit to log redoximorphic features, restrictive layers and any seasonal water table. A perc rate with no soil profile behind it is a number without a witness.
- Locate holes across the primary and reserve footprints, not clustered where digging is easy
- Auger or hand-dig to proposed trench invert depth, then scarify the smeared sidewall
- Bed the base with clean coarse gravel to prevent scour during refill
- Presoak for the duration the local health authority specifies for that soil texture
- Refill and time the drop over the required intervals, logging every reading with the hole ID
- Open a deep observation pit alongside to record horizons, mottling and any restrictive layer
Turning minutes per inch into square feet
Between the perc rate and the trench schedule sits a long-term acceptance rate — the gallons per square foot per day that soil will keep taking once a biomat has established at the infiltrative surface. Every jurisdiction publishes its own conversion, and the tables genuinely differ between neighbouring counties, so quoting a loading rate from memory or from another state's manual is a route to a rejected permit. The governing document is the state or local onsite sewage code; the EPA Onsite Wastewater Treatment Systems Manual is useful background for why the tables look the way they do, not a substitute for them.
Run the derivation for both the primary field and the reserve at the same sitting. Reserve area is not a courtesy — most codes require a designated, undisturbed replacement footprint, and discovering after excavation that the reserve only fits at a slower perc hole means redesigning the primary too. Note also that the same rate produces different trench counts depending on whether sidewall area counts toward the total, which again is a local ruling.
Convert the governing percolation rate and design flow into required absorption area and trench footage before committing a layout to paper.
Your household's estimated daily wastewater flow, in the unit shown.
How quickly water drains through your soil, from a percolation ('perc') test.
Estimated drain field area needed
670 sq ft (estimated absorption area)
This is a very general planning estimate using broad percolation categories — actual drain field design requires a licensed soil evaluator's specific percolation test results and must follow your local health department's exact sizing tables and setback requirements before any permit is issued.
They open the calculator with your figures already in it
Septic Drain Field Sizing Calculator: 667 sq ft (estimated absorption area) — 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
- This does not replace a percolation test, and it applies none of the site checks that decide whether a conventional gravity field is allowed at all — depth to the seasonal high water table, to bedrock or to a hardpan layer, and the vertical separation required beneath the trench bottom. The permitted area is whatever the governing health authority's own loading table returns for a measured rate, and that table overrides this figure.
- The four soil options each carry one flat loading rate, so every measured rate inside a band returns the same area and the answer only moves when a band edge is crossed. Nothing bounds the extremes either: 'fast draining' has no lower limit, and the 'very slow' option still returns a number for soil beyond 60 min/in, where a conventional gravity field frequently is not the design that gets permitted.
- The figure is infiltrative surface — trench-bottom or bed area on undisturbed soil — and nothing more. It carries no trench width, length, spacing or lateral count, no stone or chamber depth, and no distribution or dosing arrangement, and it applies none of the area credits some jurisdictions allow for sidewall or for chamber and gravelless products. It also excludes the reserve area and the setbacks from wells, water bodies, property lines and foundations that decide whether the field fits the lot.
- Daily flow enters as one flat average. No peaking factor is applied, and nothing adjusts for a garbage disposal, water softener backwash, a hot tub, seasonal or intermittent occupancy, or clear-water sources leaking into the system. Most codes set design flow from bedroom count rather than from metered use, so entering a low real-world figure does not earn a smaller field.
- Nothing here accounts for how the field ages or how the ground is treated. The biomat that forms at the infiltrative surface, effluent quality from a tank with no working filter or an overdue pump-out, slope and landscape position, depth of cover, and compaction from plant or construction traffic all change what the soil will keep accepting, and none of them is an input.
Design flow — the other term in the multiplication
Area is a product of two things, and the rate is only one of them. Design daily flow is fixed by bedroom count, fixture units, or occupancy depending on the code in force, and it is a design figure rather than a measured one — deliberately conservative, because a field sized on actual metered usage has no headroom when the house changes hands and the occupancy doubles. Garbage disposals, water softener discharge and high-flow fixtures push the figure up in many jurisdictions.
The tank is sized from that same flow, and it protects the rate you measured. A tank with too little liquid volume gives insufficient retention time, carries solids and grease into the laterals, and blinds the infiltrative surface faster than any soil condition would. Specify precast units to ASTM C1227 Standard Specification for Precast Concrete Septic Tanks, fit an effluent filter on the outlet tee where the code permits one, and treat the tank as the first line of defence for the drain field rather than as a separate item on the schedule.
Set tank liquid capacity from the same design flow that drives the field, so retention time protects the infiltrative surface you sized.
Bedroom count is the standard proxy for expected occupancy and wastewater flow.
Minimum recommended tank size
1,000 gallons (minimum)
This is a common general guideline — your local health department's specific code, soil percolation rate, and water-using fixture count (garbage disposal, hot tub, etc.) can all require a larger tank. Always confirm with your local permitting authority before installing.
They open the calculator with your figures already in it
Septic Tank Sizing Calculator: 1,000 gallons (minimum) — 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
- This does not replace a percolation test or soil evaluation, and tank capacity is only one line on a permit. Drain field area, depth to the seasonal high water table or bedrock, ground slope, and setbacks from wells, property lines and surface water are all outside this number, and any of them can rule out a conventional system on a site where the tank size is fine.
- Bedroom count is the only thing read. Garbage disposals, hot tubs, whirlpool baths, water softener backwash, a home business, and occupancy well above what the bedroom count implies all add flow or solids, and many jurisdictions require a larger tank for them — none of that reaches the figure shown.
- The bands are a North American health-code convention, and the metric answer is that gallon figure converted rather than a capacity drawn from a local code. Jurisdictions outside North America commonly size tanks from a population-based formula and to their own product standards, so do not hand the litre figure to a supplier as a code minimum.
- Nothing interpolates inside a band and nothing exists outside them. A one-bedroom cabin returns the same minimum as a three-bedroom house, five- and six-bedroom homes return an identical figure, and there is no answer at all above eight bedrooms.
- This is a liquid capacity minimum, not a tank specification. Compartment count, effluent filter, inlet and outlet tee and baffle detail, burial depth and traffic or buoyancy rating, access risers, and the concrete or plastic product standard are separate decisions — as is everything downstream of the tank, including drain field area, distribution box and any pump chamber.
Fitting a derived area onto a real lot
Once the square footage exists, the constraint shifts from soil to geometry. Trenches run on contour, level along their length, with the spoil and traffic kept downslope. On sloping ground the vertical separation between trenches has to be enough that the upper trench does not discharge into the lower one, and serial distribution with drop boxes or relief lines is normally how that is handled. Survey the contours properly rather than eyeballing the fall — a lateral that is out of level over its length loads its low end and starves the rest, which is a partial field pretending to be a whole one.
Setbacks then eat the plan. Distances to wells, property lines, foundations, cut banks, surface water and buried services are all code-specified and none of them are negotiable on site. Wells and cut slopes are usually the binding pair on a tight lot, and the reserve area has to clear them as well.
Materials should match what the design assumed. Perforated distribution pipe to ASTM F810 Standard Specification for Smoothwall Polyethylene (PE) Pipe for Use in Drainage and Waste Disposal Absorption Fields, or an equivalent the local code names, laid perforations-down in washed drain rock, covered with a permeable barrier that stops fines migrating into the voids without sealing the trench. Solid building sewer and tank-to-distribution pipe are a different specification entirely — ASTM D3034 Standard Specification for Type PSM Poly(Vinyl Chloride) (PVC) Sewer Pipe and Fittings is common — and mixing the two up is a defect an inspector finds in seconds.
Stone volume follows trench length, width and the depth above and below the pipe, and it is the delivery most likely to arrive short on a field of any size.
Length of the area to cover.
Width of the area.
Depth of the gravel layer.
Extra to order because loose gravel compacts down.
Gravel required
6.59 US short tons
Weighs the loose load at a typical loose bulk density of 1600 kg/m³ (100 pcf) for crushed stone or gravel as delivered. Ask your supplier for the loose density of the specific grade — a dense basalt and a light limestone differ by 15%.
- Finished volume
- 4.07 yd³
- Loose volume to order
- 4.89 yd³
- Cubic yards
- 4.89 yd³
- US short tons
- 6.59 tons
They open the calculator with your figures already in it
Gravel Calculator: 6.59 US short tons — 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
- Length × width × depth is a single rectangle at a single thickness, so a drive that curves, splays at the entrance or opens out into a parking bay has to be measured as several rectangles and the answers added, and nothing is deducted for a manhole, a kerb line or a planted island sitting inside the area.
- That thickness is then spread evenly over the whole footprint, which assumes ground already graded true — ruts, hollows and the first load that disappears into a soft patch are material the product of three numbers cannot see, and there is no separate term for a levelling course under the stone.
- The compaction allowance is one flat percentage of the finished volume, applied to the whole job at once and stopping at 40%, rather than a figure derived from the grading or the moisture of what you are actually buying; it is also added to the finished volume rather than divided out of it, so 20% here means 1.20 times the finished figure, not the 1.25 that losing a fifth of a loose load to settlement would call for.
- The weight and the loose volume are the same load: the tonnage is the loose volume to order, compaction allowance included, at one fixed loose bulk density of 1600 kg/m³ (100 pcf), with no field to change it. That suits stone delivered loose and spread or lightly rolled; a dense-graded base (crusher run, DGA, Type 1) specified by its compacted thickness weighs about 2200 kg/m³ (137 pcf) in place and is better worked on the gravel base tonnage page, which takes a compacted density.
- Nothing in the arithmetic asks what will drive over the surface or what lies beneath it: any depth from 10 mm (0.39 in) to a metre is accepted without a check on wheel loads or subgrade strength, and a build that puts a coarse base course under a finer wearing course is two separate runs of this page rather than one.
- What comes back is an exact arithmetic tonnage, not an order quantity — it is not rounded up to whole bulk bags, to the minimum load a tipper will bring out, or to the increment your yard sells in, so the last step of rounding upward is yours once you know how the material is sold.
Excavation is where the measured rate gets destroyed
Soil that percolated at 25 minutes per inch during the test can be effectively impermeable by the time the stone goes in. Three mechanisms do it: smearing from a bucket working wet soil, compaction from tracked plant driving over the footprint, and rain falling into an open trench and puddling the base. All three are avoidable and none are repairable — you cannot un-compact a trench bottom, you can only overexcavate and start lower, which changes the soil horizon you are relying on.
Fence the field and reserve before any plant arrives and keep every machine out of both, including during house construction. Excavate only in dry conditions, rake or scarify the trench bottom by hand immediately before placing stone, and never leave an open trench overnight under a forecast. Compaction of the surrounding fill matters too, but for the cover rather than the infiltrative surface — over-compacted cap material sheds nothing and traps everything, while loose cover settles into a depression that ponds surface water directly over the laterals. Where a compaction spec applies to the cap or an access drive, ASTM D698 Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Standard Effort provides the reference density the field results are compared against.
Trench walls that have to be laid back change the footprint the field needs on the lot, which is a different number from the absorption area itself.
Depth from existing ground to the trench bottom.
Width needed at the base for the work.
Run of trench at this profile, for the total excavation volume.
This calculator does not classify soil. A competent person must.
Excavation top width
19.5 ft
Geometry at the tabulated maximum allowable slope for the soil type you selected. It is not a classification, and it is not a protective-system design.
- Setback each side
- 8 ft
- Slope angle from horizontal
- 45 °
- Spoil volume per unit length of trench
- 3.41 yd³/ft
- Extra volume caused by sloping
- 2.37 yd³/ft
- Top width in feet
- 19.5 ft
- Total bank excavation volume
- 112.44 yd³
They open the calculator with your figures already in it
Trench Sloping & Excavation Width Calculator: 19.5 ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
The top width is what the excavation costs you at the surface, and it is the figure people are surprised by: in Type C soil a trench 2.4 m deep opens 7.2 m wide before you have dug anything. The section is drawn to one scale on both axes, so the batter you see is the batter the soil classification requires.
What this calculation does not cover
- Soil classification must be made on site by a competent person using the tests in OSHA Appendix A. This calculator applies the slope for a classification you supply; it cannot make one.
- Excavations deeper than 20 feet (6.096 m) require a protective system designed by a registered professional engineer. The depth input is capped for that reason.
- Layered soils are classified by the weakest layer present, and previously disturbed ground is Type C regardless of appearance.
- Spoil must be kept at least 2 ft (0.6 m) from the edge, and that setback is not included in the top width here. Surcharge from spoil, plant or adjacent structures changes the problem entirely.
Reading a field whose rate is going
Failure announces itself before it surfaces. Slow drains and gurgling fixtures with a full tank point at the outlet side; a lush green stripe over the first lateral in dry weather means effluent is rising toward the surface at the near end; soggy ground or breakout at the toe of a slope means the field is full and the soil has stopped accepting. Distinguish hydraulic overload — too much water for the area that was built — from biomat blinding, because the remedies differ and only one of them is a plumbing fix.
Pull the tank lids and check the sludge and scum depths before touching the field. A tank that has not been pumped in years and has lost its outlet baffle has been feeding solids to the laterals, and until that is corrected any field work is temporary. Where genuine overload is the cause, the honest answer is usually the reserve area, sized from the same governing perc rate, with the original field rested rather than abandoned.
| What you see | What it usually means | Check first |
|---|---|---|
| Fixtures slow, tank level normal | Blockage between house and tank | Building sewer, inlet baffle |
| Tank level above outlet invert | Field not accepting, or line blocked | Outlet tee, effluent filter, distribution box |
| Green stripe over first lateral only | Uneven loading, near-end saturation | Lateral level, distribution box levelling |
| Wet ground at toe of slope | Field at capacity or shallow restrictive layer | Deep pit log, original perc data, flow records |
| Backup after heavy rain only | Surface or groundwater entering the system | Cap grading, downspouts, curtain drain |
The record that makes the next decision cheap
Close the job with a set of documents that lets somebody solve a problem in fifteen years without re-excavating to find out what is down there. That means the perc logs with hole locations and depths, the soil profile description, the loading rate applied and the code edition it came from, an as-built showing tank, distribution box and every lateral dimensioned from two fixed points, and the reserve area marked on the plot plan.
Tell the homeowner what protects the rate: pumping intervals appropriate to tank size and household, nothing driving or parking on the field, no trees or irrigation over the laterals, and roof and surface drainage kept away from the footprint. Where an advanced treatment unit was fitted to compensate for marginal soil, note the certification it was supplied under — NSF/ANSI 40 Residential Wastewater Treatment Systems, or NSF/ANSI 245 where nitrogen reduction was the reason — along with its service requirement, because an unserviced unit reverts the whole design to a soil condition that was never adequate on its own.
Before the excavator books in
Percolation data governs the whole schedule, so nothing below gets ordered until the governing rate and the design flow are both settled and the local loading table has been read.
- Percolation logs, all holes, primary and reserve — Depths at proposed invert, presoak duration recorded, holes located on the plot plan
- Deep observation pit profile — Horizons, mottling, restrictive layer and any seasonal water table logged before design
- Local loading-rate table and setback schedule — Current edition from the governing health authority — neighbouring counties differ
- Perforated distribution pipe and washed drain rock — Perforations down, permeable barrier over stone, no fines in the voids
- Precast tank with effluent filter and sound baffles — Sized from the same design flow as the field; protects the infiltrative surface
- Exclusion fencing around field and reserve — Up before any plant arrives and left until landscaping is finished
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.