Greywater Soil Percolation Calculator — Test & Size Your Basins

How to Run the Percolation Test

Dig the test hole at your planned basin location. Make it 12 inches deep and about 6 inches wide. Use a shovel or post-hole digger.
Pre-soak the hole by filling it completely with water. Let it drain completely. This saturates the soil to simulate post-laundry conditions. Wait for it to empty (may take 15 minutes to several hours).
Refill the hole completely with water and start your timer at the same moment.
Mark the water level at the start using a stick, pencil, or tape on the hole wall.
After 30 minutes, measure how many inches the water level dropped. If the hole drained completely before 30 minutes, note the time it took to fully drain.
Record your result: either (a) inches dropped in 30 minutes, or (b) total minutes to drain completely.

Enter Your Perc Test Results

How do you want to enter your result?

Inches of water level drop in 30 minutes If fully drained before 30 min, use the "minutes to drain" option instead

Minutes for hole to fully drain 12 inch hole. If still draining after 4 hours, enter 240+

Daily greywater volume to absorb (gallons) Use the sizing calculator to find your daily volume first

Number of irrigation zones

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Run the field test first

Follow the 6-step perc test on the left, then enter your results to get exact basin dimensions.

Your Soil Profile & Basin Specifications

Soil classification—

Absorption rate—

Daily volume per zone—

Required area per zone—

Basin diameter (circular)—

OR basin length × width—

Recommended basin depth—

Mulch cap thickness4 inches minimum

Total mulch cubic feet—

Perc Rate → Soil Type Reference

Drop in 30 min	Soil Type	Basin Factor
> 6 inches	Sandy / gravel	0.3 sq ft per gpd
3–6 inches	Sandy loam	0.45 sq ft per gpd
1–3 inches	Loam / silt loam	0.6 sq ft per gpd
0.25–1 inch	Clay loam	0.85 sq ft per gpd
< 0.25 inch	Clay / caliche	1.25 sq ft per gpd
No drainage 4+ hrs	Hardpan / impervious	Not suitable ⚠️

💡 Do Multiple Perc Tests

Soil varies across a property. If you have 4 planned irrigation zones, run a perc test at 2–3 of the locations before finalizing sizes. A zone in a shaded area with organic-matter-rich soil might drain twice as fast as a zone in sunny clay-heavy ground on the same lot. Size each zone based on its actual soil rather than a single property-wide estimate.

Percolation testing and basin sizing calculations are estimates for planning purposes. Actual soil absorption varies with moisture content, compaction, and seasonal groundwater levels. When in doubt, build larger basins than calculated and monitor performance after the first season of use.