Seepage Pit Calculator
Size a seepage pit (soak pit) for septic tank effluent: daily wastewater, percolation test rate, required sidewall absorption area, effective depth for your pit diameter and number of pits.
How it works
flow = people × litres per person · absorption rate (gal/ft²/day) = 5 ÷ √(perc minutes per inch) · area = flow ÷ rate · effective depth = area ÷ (π × diameter)
A seepage pit (soak pit, pozo de absorción) disperses septic-tank effluent through its sidewalls; the bottom is not counted because it clogs first. The US Public Health Service Manual of Septic-Tank Practice relates the percolation test to the allowable loading with Q = 5 ÷ √t gallons per sq ft per day — 1.6 at 10 min/inch, 0.9 at 30 min/inch — and does not recommend pits in soils slower than 30 min/inch. The effective sidewall is measured below the inlet, using the outside diameter including the gravel ring, and pits should be at least three diameters apart. Always check local rules on groundwater depth and well distances.
Worked example
5 people at 150 L a day produce 750 L. A perc test of 10 min/inch gives 5 ÷ √10 = 1.58 gal/sq ft a day (64.4 L/m²), so the pit needs 750 ÷ 64.4 = 11.6 m² of sidewall. A 1.5 m diameter pit gives π × 1.5 = 4.71 m² per metre of depth, so one pit needs 2.47 m of effective depth below the inlet — within the 3 m limit.
Sources: US PHS — Manual of Septic-Tank Practice (1967) · US EPA — Onsite Wastewater Treatment Systems Manual (2002)
Frequently asked questions
How deep should a seepage pit be?
Deep enough that the sidewall below the inlet provides the needed absorption area: area ÷ (π × diameter). For 5 people on soil that percolates at 10 min/inch, a 1.5 m (5 ft) pit needs about 2.5 m (8 ft) of effective depth. Keep the bottom at least 1.2 m (4 ft) above the groundwater.
How do I do a percolation test?
Dig a hole about 30 cm (12″) across to the depth of the pit’s absorbing layer, soak it for several hours, then refill to 15 cm (6″) and time how long the water takes to drop 2.5 cm (1″). Repeat until two readings agree.
What soil is too slow for a seepage pit?
Soils that take longer than 30 minutes per inch (about 12 min/cm) — clays and silty clays — should not use seepage pits. A larger drain field, a mound or an engineered system is needed.
How far apart should two seepage pits be?
At least three times the diameter of the larger pit, and never less than 6 m (20 ft) for very deep pits, so the soil between them is not overloaded.
Related calculators
Septic systems must meet local health-department rules and soil tests; use this as a first estimate, not a permit design.