How this is calculated
The lookup runs on a national dataset shipped with the page. Each rated soil map unit carries an interpretation called ENG — Septic Tank Absorption Fields, grading the soil as not limited, somewhat limited or very limited and naming the features behind the grade. County acres were weighted by each soil component's share of its map unit, then summed. The unrated share is water, quarries and urban land, held apart.
The sizing panel runs the arithmetic a designer would. Design flow is bedrooms times gallons per bedroom per day. Required absorption area is that flow divided by the application rate the code assigns to your perc rate, and trench run is the area divided by the trench width, capped at two feet for design.
Worked example. A three-bedroom house with modern fixtures is 3 × 110 = 330 gallons per day. A measured 25 min/in falls in the 21–30 band, where the application rate is 0.60 gallons per day per square foot. So 330 ÷ 0.60 = 550 ft² of trench bottom, and 550 ÷ 2 = 275 feet of trench — which is exactly what New York's printed Table 4A gives for that flow and band. Virginia's Table 5.4 lists 158 square feet per 100 gallons at 25 min/in with gravity distribution, which is 521 ft² for the same house — two codes, different arithmetic, within 5% of each other.
What the soil survey rates, and where it stops
The rating judges how much trouble the soil will give a buried trench taking septic tank effluent every day for decades.
The features named in the lookup are the specific rules that fired. Ponding and a water table near the surface mean the trench sits in water part of the year, and slow water movement means the effluent backs up.
Seepage below the trench is the opposite problem: water leaves so fast the soil never treats it, which is why a code can refuse sand as firmly as clay. Depth to bedrock, slope, large stones, cemented pans and organic subsidence each cut the usable soil column from a different direction.
Read the rating against the country
Across the survey as queried in September 2026, 85% of rated US soil is very limited for a septic absorption field, 13% somewhat limited and 2% not limited. In 2,891 of the 3,158 counties, at least 60% of the rated land is very limited. Most working American septic systems therefore sit on soil the survey calls very limited. The rating flags design difficulty and cost, and a map unit can still hold a dry knoll the survey never separated.
The federal onsite wastewater manual pushes the same way. Its 2002 edition states that soil structure, texture and consistence are better predictors of hydraulic capacity than percolation tests, and that most states have added a soil profile description alongside the holes.
The perc test itself
A percolation test is a hole of specified depth, presoaked so the soil is wet, refilled with clean water and timed as the level falls. The result is minutes per inch of drop. New York requires at least two tests per proposed system, more holes where the soil profiles differ, and the more restrictive result governs the design.
Who holds the shovel varies by state. Pennsylvania puts a certified sewage enforcement officer in charge of conducting or confirming every test used to judge a site's suitability, and gives the local agency 20 working days to finish testing once an application lands. New York leaves the work to the local health department, which may accept other soil tests in place of the perc test.
Cost splits in two. The government side is published and small: Virginia's statutory filing fee for an onsite sewage system permit application is 75 dollars, plus 75 dollars for inspections on alternative discharging systems. The private side is the backhoe, the evaluator's hours and any engineered design, quoted locally, and that is the part that moves.
The two tables the sizing panel applies
New York states an application rate directly. Pennsylvania states an area factor instead, and runs further into slow soils by demanding a pressure-dosed sand filter or mound. Values sit at the slow end of each band, which is the conservative read.
| Perc rate (min/in) | NY rate (gpd/ft²) | NY trench, 3 bed (ft) | PA standard (ft²/gpd) | PA mound or sand filter (ft²/gpd) |
|---|---|---|---|---|
| Under 1 | Unsuitable | — | Unsuitable | Unsuitable |
| 1–2 | 1.20 | 138 | Unsuitable | Unsuitable |
| 3–5 | 1.20 | 138 | Unsuitable | 1.50 |
| 6–7 | 1.00 | 165 | 1.19 | 1.50 |
| 8–10 | 0.90 | 184 | 1.19 | 1.50 |
| 11–15 | 0.80 | 207 | 1.19 | 1.50 |
| 16–20 | 0.70 | 236 | 1.39 | 1.50 |
| 21–30 | 0.60 | 275 | 1.79 | 1.50 |
| 31–45 | 0.50 | 330 | 2.24 | 1.89 |
| 46–60 | 0.45 | 367 | 2.66 | 2.22 |
| 61–90 | Outside the range | — | 3.35 | 2.82 |
| 91–120 | Outside the range | — | Unsuitable | 3.33 |
| 121–150 | Outside the range | — | Unsuitable | 3.97 |
| 151–180 | Outside the range | — | Unsuitable | 4.62 |
| Over 180 | Outside the range | — | Unsuitable | Unsuitable |
The New York trench column is a three-bedroom house at 110 gpd per bedroom, read straight from Appendix 75-A Table 4A; the rate column is Table 4B. Pennsylvania columns are 25 Pa. Code § 73.16 Table A, evaluated at the slow end of each New York band. Every Pennsylvania mound and sand filter entry carries footnote A, pressure dosing required, as does the standard column at 61–90 min/in. Past 90 min/in the last column is an elevated sand mound only, because footnote D rules out a subsurface sand filter there.
What a failed perc actually means for the build
It means a different machine, a bigger footprint and a higher bill. Four routes exist, and the county decides which are open.
- Raised or mound system. Trenches built in imported sand above natural grade. New York allows one where the original soil above the restrictive layer is one to two feet deep and percolates faster than 60 min/in, groundwater sits at least a foot below the surface and the slope is no steeper than 15%. The basal area is sized at 0.2 gallons per day per square foot.
- Pressure-dosed sand filter or mound. Effluent is dosed under pressure through a sand bed before it reaches the soil. Pennsylvania runs a subsurface sand filter to 90 min/in and an elevated sand mound to 180 min/in, long after a standard trench is refused.
- Sealed holding tank. No treatment, no field, a pump truck on a schedule; often permitted only for a replacement.
- Composting toilet with a graywater field. This changes the arithmetic rather than the machine: New York's design flow drops from 110 to 75 gallons per day per bedroom with waterless toilets, about a third less absorption area for the same house.
That last line is the one worth carrying into a land offer: cutting design flow shrinks every downstream number, and it is the cheapest lever on a marginal site. Our composting toilet comparison covers living with one, and the off-grid sanitation guide walks the decision from flush to field.
Quick answers
What does it mean when land will not perc?
The soil moves water too slowly, or too fast, for a conventional buried drainfield at the area the code requires. Pennsylvania refuses a standard trench slower than 90 min/in or faster than 6 min/in, and rates anything faster than 3 min/in unsuitable for every system in its table.
Can I check land before I buy it?
You can screen it here, using the soil survey's own septic rating and the features behind it. That is a map-unit judgment and never replaces the county's test hole on your parcel.
What percolation rate is good?
Roughly 6 to 30 min/in is the comfortable middle. New York accepts 1 to 60 min/in for a conventional trench and rejects anything faster, because untreated effluent reaches groundwater.
Who performs a perc test?
It depends on the state: a certified sewage enforcement officer in Pennsylvania, the local health department in New York. Virginia's statutory application fee is 75 dollars, with the digging quoted separately.
What are the options if the land fails?
A mound, a pressure-dosed sand filter, a sealed holding tank, or a composting toilet with a small graywater field.
Does a composting toilet shrink the field?
Yes. New York's design flow falls from 110 to 75 gallons per day per bedroom with waterless toilets, cutting the absorption area by about a third.
Sources
County ratings computed from the USDA Natural Resources Conservation Service Soil Data Access service (SSURGO), interpretation ENG — Septic Tank Absorption Fields, queried September 2026, weighted by component percentage and county overlap acres. Sizing tables from New York State Sanitary Code Appendix 75-A, Tables 1, 4A and 4B, and 25 Pa. Code § 73.16 Table A, cross-checked against Virginia 12VAC5-610-950 Table 5.4, which states area in square feet per 100 gallons. Sewage enforcement officer duties from 25 Pa. Code § 72.41 and the 20-working-day limit from § 72.42. Permit fee from Code of Virginia § 32.1-164. Soil morphology guidance from the USEPA Onsite Wastewater Treatment Systems Manual, EPA/625/R-00/008, 2002.
Go deeper
Water arrives before it leaves, so size the supply side with the water and rain-catchment calculator, which turns roof area and rainfall into tank size. Then read the off-grid land buying checklist for the other tests worth ordering before closing, and choosing your off-grid water source. The land price atlas pairs with this one when a cheap parcel needs a sanitation budget attached.