Calculator
Percolation test calculator
Enter your drop reading and interval. You get the minutes-per-inch rate, the soil class it matches, the loading rate your drainfield would be sized on, and whether a conventional field is realistic.
Percolation rate
30 min/inch
60 minutes ÷ 2 inches of drop. Suitable
Loading rate
0.5
gpd per ft²
Absorption area
720
ft² for a 3-bedroom home
Trench length
240 ft
3 × 80 ft at 3 ft wide
With reserve
1,440
ft² of protected yard
- Consistent with sandy loam at a 0.5 gpd/ft² loading rate.
- Hand-texture cross-check — sandy loam should feel like this: Gritty with some smoothness; ball holds, ribbon <1 in.
How to run a percolation test
- Dig the holes. 6 to 12 inches across, down to the proposed trench depth — usually 18 to 36 inches. Most codes require at least three holes spread across the proposed field area, not clustered in one corner.
- Scarify the sides. Digging smears the soil and seals the hole. Scratch the sides with a knife or nail and clear the loose material out of the bottom.
- Add gravel. Two inches of coarse gravel in the bottom stops the falling water from scouring a crater.
- Presoak. Keep the hole filled with water for at least 4 hours, and 24 hours in clay. This is the step amateurs skip and it is the step that matters most — unsaturated soil percolates several times faster than soil in service.
- Take timed readings. Refill to a set depth and measure the drop over a fixed interval. Repeat until two consecutive readings agree within about a sixteenth of an inch.
- Divide. Minutes ÷ inches of drop = minutes per inch.
What the number means
| Perc rate | Soil | Loading rate | What it means |
|---|---|---|---|
| Under 1 min/in | Gravel or coarse sand | — | Too fast Effluent passes through untreated. Expect a sand-lined trench or an advanced treatment unit. |
| 1–5 min/in | Coarse sand | 0.80 | Marginal Fast. Some states require a liner or extra separation. |
| 6–20 min/in | Fine sand, loamy sand | 0.60 | Excellent The easiest soil to permit and the cheapest to build in. |
| 21–30 min/in | Sandy loam | 0.50 | Good Conventional trenches, moderate field size. |
| 31–45 min/in | Loam | 0.40 | Good Workable; field roughly 25% larger than sandy loam. |
| 46–60 min/in | Silt loam | 0.30 | Fair Large field. Check whether your county still allows gravity trenches. |
| 61–120 min/in | Clay loam | 0.15–0.20 | Poor Very large field or an alternative system. |
| Over 120 min/in | Clay | — | Unsuitable Conventional trenches are not permitted. Mound, at-grade, or drip. |
Soil texture as a cross-check
A perc test measures one afternoon. A texture classification describes the soil's permanent behaviour, which is why most modern codes now prefer it. You can run the hand-texture test yourself with a handful of moist soil and compare it against your perc result — if they disagree badly, the perc test is usually the one that is wrong.
| Texture | How it feels when moist | Loading rate |
|---|---|---|
| Coarse sand / gravel | Gritty, falls apart when wet, will not hold a ribbon. | 0.8 gpd/ft² |
| Medium sand | Gritty and loose; individual grains visible. | 0.8 gpd/ft² |
| Fine sand | Gritty but softer; barely holds shape when squeezed. | 0.6 gpd/ft² |
| Loamy sand | Mostly gritty; forms a weak ball that crumbles easily. | 0.6 gpd/ft² |
| Sandy loam | Gritty with some smoothness; ball holds, ribbon <1 in. | 0.5 gpd/ft² |
| Loam | Even mix — slightly gritty, slightly smooth, slightly sticky. | 0.4 gpd/ft² |
| Silt loam | Floury and smooth when dry; ribbon under 1 inch. | 0.3 gpd/ft² |
| Sandy clay loam | Gritty but sticky; ribbons 1–2 inches before breaking. | 0.25 gpd/ft² |
| Clay loam | Sticky and plastic; ribbons 1–2 inches. | 0.2 gpd/ft² |
| Silty clay loam | Smooth and sticky; ribbons about 2 inches. | 0.15 gpd/ft² |
| Clay / silty clay | Very sticky and plastic; ribbons over 2 inches. | 0.1 gpd/ft² |
Common questions
What is a good perc rate?
Between about 10 and 45 minutes per inch is the sweet spot: fast enough to accept effluent, slow enough to treat it. Under 5 mpi the soil drains too fast to treat properly and many codes require a sand liner. Over 60 mpi you are heading toward a mound or drip system, and over 120 mpi a conventional trench field is generally not permitted.
Can I do my own perc test?
You can do one to inform your own decision — for example, before buying a lot — and it is worth doing. But no health department will accept it. The official test must be run by a licensed soil evaluator, engineer, or health department staff, usually with multiple holes, a presoak period, and witnessed readings.
Why does my result differ from the official one?
Almost always because of soil moisture and presoak. A test run on dry summer soil percolates far faster than the same hole in March. That is why codes require a presoak — often 24 hours — and why many states schedule testing for the wet season.
Has the perc test been replaced?
Increasingly, yes. Many states now prefer a soil morphology evaluation, where a soil scientist examines a pit and classifies texture, structure, and redoximorphic features to assign a loading rate directly. It is more reliable than a perc test because it reads the soil's long-term behaviour rather than one afternoon's infiltration.
My lot failed the perc test. Is it unbuildable?
Not usually. It means a conventional trench field will not work, which pushes you toward a mound, at-grade, drip, or aerobic system — more expensive, but almost always possible. The exceptions are sites with bedrock or water table too close to the surface to meet the vertical separation requirement at all.
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