Reading your result
What changes the area you need
Two inputs drive it: your daily design flow (150 gallons per bedroom) and how fast the soil drains. The loading rate is 1.2 ÷ √T gal/ft²/day and the required area is design flow ÷ loading rate — so slower soil (a higher perc number) means a bigger field. Most codes also require a 100% reserve area set aside for future replacement. No perc number yet? Start with the perc rate calculator.
Typical perc rates by soil type
| Soil type | Typical perc rate |
|---|---|
| Sand | ~5 min/in (fast) |
| Sandy loam | ~15 min/in |
| Loam | ~30 min/in |
| Clay loam | ~45 min/in |
| Clay | ~90 min/in (often too slow) |
Full guide: Reading your perc test results → — soil classes, the loading-rate formula, and when slow soil forces an alternative system.
Frequently asked questions
How do I size a septic drain field?
Divide the daily design flow by the soil loading rate. The loading rate comes from the perc rate: loadingRate = 1.2 ÷ √T, where T is minutes per inch. Faster soils carry more wastewater per square foot, so they need less area.
How big a leach field do I need for a 3-bedroom house?
A 3-bedroom home has about a 450 GPD design flow. In medium loam soil (T ≈ 30) the loading rate is about 0.22 gal/ft²/day, so you need roughly 2,050 ft² of absorption area.
What is a good perc rate for a drain field?
Conventional gravity fields work well between about 5 and 60 minutes per inch. Above ~60, you usually need a mound, sand filter, or aerobic unit; below ~1, the design may need modification to protect groundwater.
How many trenches will I need?
Divide the absorption area by the trench width (commonly 2 feet) to get total trench length, then split that into runs — most fields use multiple 50–100 ft laterals fed by a distribution box.
Do I need a reserve area?
Most codes require a reserve (replacement) area equal to 100% of the primary field, kept clear for the day the original field needs replacing. Plan for it even though this tool sizes the primary field.
Is a drainfield the same as a drain field or leach field?
Yes — "drainfield" (one word), "drain field" (two words), "leach field," "absorption field," and "disposal field" all mean the same thing: the network of perforated pipe and gravel or chambers that disperses treated effluent into the soil. This drainfield size calculator works the same whichever term your local code uses.
Sources & methodology
- US EPA — Onsite Wastewater Treatment Systems Manual (EPA/625/R-00/008): 60 gal/person/day design flow
- Soil loading rate from percolation rate (1.2 ÷ √T) — EPA / Wisconsin onsite design model
- Your state or county health department — the authoritative source for local code, minimums, setbacks, and permits
Estimates compiled from the sources above and standard engineering formulas — not engineering, professional, or legal advice, and may not reflect your local code. See our full methodology and disclaimer.