Percolation Rate to Soil Loading Rate
Convert your percolation test result (minutes per inch) into a soil loading rate (gallons per day per square foot) to size a drain field.
Calculator
A perc rate of 30 min/inch falls in the 0.50 gpd/sq ft loading band. This is a labeled planning band — your local health department’s own perc → loading table governs the design, and very slow soil may not perc at all.
Before anyone sizes a drain field, the soil has to be tested. The classic method is a percolation (“perc”) test: dig test holes, presoak them, then time how many minutes it takes the water level to fall one inch. A fast drop (few minutes per inch) means sandy, permeable soil that can absorb a lot of effluent per square foot; a slow drop means tight clay that absorbs little, so you need a much larger field — or, if it is slow enough, an engineered alternative system.
This tool turns your perc number into the soil loading rate the sizing math uses. It is the front door to the whole sizing chain: loading rate → absorption area → trench length & laterals. The guide what is a perc test and how to read it explains the field procedure; the perc-to-loading table lists the bands.
Formula
A percolation test times how long water takes to drop one inch in a soaked test hole; the result in minutes per inch maps to a soil loading rate band:
loading rate (gpd/sq ft) = band( perc minutes per inch )
- ≤ 5 min/inch (fast, sandy) → 1.2 gpd/sq ft
- 6–30 min/inch (moderate) → 0.5 gpd/sq ft
- 31–60 min/inch (slow) → 0.2 gpd/sq ft
- > 60 min/inch (very slow) → lowest band, or the soil fails
Slower soil (more minutes per inch) means a lower loading rate, which means a bigger field.
Worked example
A perc test that takes 30 minutes for the water to drop one inch:
30 min/inch → 0.5 gpd/sq ft
Carry that into absorption area: a 300 gpd home on this soil needs 300 ÷ 0.5 = 600 sq ft of field. A faster 5 min/inch result would give 1.2 gpd/sq ft and only 250 sq ft; a slow 45 min/inch result gives 0.2 gpd/sq ft and 1,500 sq ft. The perc test is the biggest single lever on how much drain field you have to build.
Perc tests, soil evaluation and what a failed test means
The bands here are a widely used planning approximation. In practice your health department publishes its own perc-to-loading table, and many jurisdictions have moved to soil morphology (a licensed soil evaluator classifying texture and structure in a dug pit) either alongside or instead of the timed perc test, because soil description predicts long-term acceptance better than a single water-drop timing. Either way the principle is the same: coarser, more permeable soil gets a higher loading rate and a smaller field.
A few practical points. Perc rates that are too fast (well under 1 min/inch, pure gravel or sand) can also fail, because effluent races through before the soil can treat it. Tests run when the ground is saturated in spring often perc slower than in a dry summer, so timing matters. And a failed perc is not the end — it usually means a mound, at-grade or aerobic system rather than a conventional gravity field. The test result, the loading table and the design must be verified by a licensed septic designer and approved by your local health department.
Reference table
| Percolation rate | Soil | Loading rate |
|---|---|---|
| ≤ 5 min/inch | Fast (sandy) | 1.20 gpd/sq ft |
| 6–30 min/inch | Moderate (loam) | 0.50 gpd/sq ft |
| 31–60 min/inch | Slow (clay/silt) | 0.20 gpd/sq ft |
| > 60 min/inch | Very slow | 0.20 gpd/sq ft (or fails) |
Labeled planning band — your local health department and a licensed septic designer set the values that actually govern your permit.
Frequently asked questions
What is a good perc rate for a septic system?
Roughly 1 to 30 minutes per inch is comfortable for a conventional gravity drain field. Faster than about 1 min/inch can be too fast to treat effluent; slower than 60 min/inch usually needs an engineered or alternative system.
How do I read a percolation test result?
The result is minutes per inch — how long the water took to drop one inch in the presoaked hole. Fewer minutes = faster soil = higher loading rate = smaller field. This tool converts it to the gpd/sq ft loading rate.
What loading rate goes with 30 minutes per inch?
A perc rate of 30 min/inch falls in the moderate band at about 0.5 gpd/sq ft. Your local health department’s own table governs the exact figure used on your permit.
What happens if my perc test fails?
Very slow soil may not support a conventional field. Options include a mound, at-grade, sand-filter or aerobic treatment system, all of which a licensed designer sizes and the health department approves. A failed perc raises cost but rarely makes a lot unbuildable.