Drainfield size calculator

The size of a drainfield comes from the water a house makes and the soil under it. This tool shows the figure for each, and its source.

How fast each soil takes water

EPA gives a loading rate for each soil texture. The rate is the gallons a square foot of trench takes in a day.

Soil	Loading rate	What the rate covers	Source
Gravel or coarse sand	0.8 gallons per square foot per day	Coarse sand or sand, single grain, no structure. Waste strength of 150 mg BOD per liter, EPA Table 4-3. This soil always forms as single grains. The manual gives it one rate, with no adjustment for structure.	U.S. EPA — Onsite Wastewater Treatment Systems Manual (EPA/625/R-00/008), Table 4-3, p. 4-12 · checked 2026-09-17
Sand	0.4 gallons per square foot per day	Fine sand or finer, single grain, no structure. Waste strength of 150 mg BOD per liter, EPA Table 4-3. The manual also covers loamy fine sand under this rate. It forms single grains, so the manual gives one rate with no adjustment for structure.	U.S. EPA — Onsite Wastewater Treatment Systems Manual (EPA/625/R-00/008), Table 4-3, p. 4-12 · checked 2026-09-17
Sandy loam	0.2 gallons per square foot per day	Sandy loam, massive and structureless. Waste strength of 150 mg BOD per liter, EPA Table 4-3. The manual allows a higher rate for the same soil with strong blocky or granular structure. Use it only when a soil scientist confirms that structure.	U.S. EPA — Onsite Wastewater Treatment Systems Manual (EPA/625/R-00/008), Table 4-3, p. 4-12 · checked 2026-09-17
Loam	0.2 gallons per square foot per day	Loam, massive and structureless. Waste strength of 150 mg BOD per liter, EPA Table 4-3. The manual allows a higher rate for the same soil with strong blocky or granular structure. Use it only when a soil scientist confirms that structure.	U.S. EPA — Onsite Wastewater Treatment Systems Manual (EPA/625/R-00/008), Table 4-3, p. 4-12 · checked 2026-09-17
Clay loam	0.2 gallons per square foot per day	Clay loam, weak blocky or granular structure. Waste strength of 150 mg BOD per liter, EPA Table 4-3. The manual rates this soil only when it has blocky or granular structure. A massive or platy profile earns no rate, so the manual finds it unsuitable.	U.S. EPA — Onsite Wastewater Treatment Systems Manual (EPA/625/R-00/008), Table 4-3, p. 4-12 · checked 2026-09-17
Clay	0.2 gallons per square foot per day	Clay, strong blocky or granular structure. Waste strength of 150 mg BOD per liter, EPA Table 4-3. This is the only structure and grade the manual rates for this soil. Every weaker structure earns no rate, so the manual finds it unsuitable for a conventional field.	U.S. EPA — Onsite Wastewater Treatment Systems Manual (EPA/625/R-00/008), Table 4-3, p. 4-12 · checked 2026-09-17

Sources

Every figure on this page has a government source and the date we checked it.

U.S. EPA — Onsite Wastewater Treatment Systems Manual (EPA/625/R-00/008), Table 4-3, p. 4-12 · checked 2026-09-17

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