Drain Field Size Calculator (absorption area by soil type)
Minimum drain field absorption area from bedrooms and soil type, using Washington State's loading-rate table.
What size drain field do I need?
Find your bedroom count and soil type in the chart. Areas are square feet of infiltrative surface for ordinary septic tank effluent (Column A).
| Bedrooms | Soil 1 | Soil 2 | Soil 3 | Soil 4 | Soil 5 | Soil 6 |
|---|---|---|---|---|---|---|
| 2 | 240 | 240 | 300 | 400 | 600 | 1,200 |
| 3 | 360 | 360 | 450 | 600 | 900 | 1,800 |
| 4 | 480 | 480 | 600 | 800 | 1,200 | 2,400 |
| 5 | 600 | 600 | 750 | 1,000 | 1,500 | 3,000 |
| 6 | 720 | 720 | 900 | 1,200 | 1,800 | 3,600 |
Worked example
A 3-bedroom house in Washington has a design flow of 3 × 120 = 360 gallons per day. On soil type 4 (sandy loam or loam) with ordinary septic tank effluent, Table VIII allows 0.6 gallons per square foot per day, so the absorption area is 360 ÷ 0.6 = 600 square feet. On soil type 2 (coarse sand) at 1.0 it would be 360 square feet, and on soil type 6 at 0.2 it would be 1,800 square feet.
How it works
Washington sizes the infiltrative surface by dividing the design flow by the maximum hydraulic loading rate for the finest-textured soil within the vertical separation. Design flow for a single-family home is 120 gallons per bedroom per day (45 gpd per person, 2 people per bedroom, plus 33% surge), with a 240 gpd minimum. Soil type 7 (clays, cemented or massive soils) is unsuitable. Both the primary and the reserve area must be sized at 100% or more of the approved loading rate, and the local health officer may require Column A for the reserve. Only a soil log by a licensed designer can set your soil type, and other states use different tables.
Questions
How big is a drain field for a 3-bedroom house?
Under Washington's rule it's 360 gpd divided by your soil's loading rate: 360 sq ft in coarse sand, 600 sq ft in loam, and 1,800 sq ft in the slowest permitted soil type 6.
Can I build a drain field in clay?
Not a conventional one in Washington. Table VIII lists sandy clay, clay, silty clay and soils with massive or strong platy structure as unsuitable.
Do I need a reserve area?
Yes. WAC 246-272A-0234(8) says both the primary and reserve areas must be sized at least 100% of the approved loading rates.
Sources
Guides
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