Drain field sizing
Drain Field Size Calculator
Combine your home size with the soil's perc rate to get the required absorption area and trench layout. Need your perc rate first? Use the perc test calculator.
9 trenches · 895 ft total · 2-ft wide
Estimate only. A licensed soil evaluator's perc test and your local code determine the legal design.
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
| 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.