What this calculator does
Where the soil drains freely, a soakaway pit is the simplest way to dispose of roof or surface water. This calculator turns the time recorded in a percolation test into a soakage rate, works out how much volume the design storm produces, and gives the base area the pit needs at your chosen depth.
How to use it
- Enter the area draining to the soakaway and the depth of the design storm.
- Enter the percolation test result in seconds per millimetre; finer soil gives a longer time.
- Enter the soakaway depth — 1.0 to 1.5 m is typical, but it must sit below the frost line.
- The square footprint follows from the storage volume divided by the depth.
Governing formula
k = 1000 / t_p ; V = A · P · C ; A_base = V / d Where:
-
t_p= Percolation test time [s/mm] -
A= Drainage area [m²] -
P= Design rainfall depth [m] -
d= Soakaway depth [m]
Derived Equations:
side = √A_base Worked example — A 150 m² roof with a 25 mm storm on sandy soil
The figures below were produced by running this page's own engine with the example inputs (Area draining to the soakaway (m²), Design rainfall depth (mm), Runoff coefficient (–), Percolation test result (s/mm), Soakaway depth (m)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Storage required | 3.56 m³ |
|---|---|
| Required base area | 2.97 m² |
| Soakage rate | 50 mm/h |
| Square pit side (square option) | 1.72 m |
| Assessment | Need ≈ 3.6 m³ storage; a 1.2 m-deep pit with ≈ 1.7 m × 1.7 m base (≈ 3 m²) suits the 50.0 mm/h soakage rate from the 20 s/mm test. Backfill with washed aggregate and geotextile. |
- A 20 s/mm test corresponds to a soakage rate of about 50 mm/h.
- The roof sheds 150 × 0.025 × 0.95 = 3.6 m³ per storm.
- At 1.2 m deep that needs a base area of about 3 m² — a 1.7 m square pit.
Assumptions & limitations
Engineering calculation. Converts a percolation test result into a soakage rate and sizes a square soakaway pit from the storage volume and depth.
- The percolation test measures surface infiltration and under-predicts performance after the ground saturates.
- No groundwater level constraint; a high water table can make the pit ineffective.
- A single test is unreliable — the local authority will normally require a soakaway test to a stated standard.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- BS EN 752-4 — soakaways and drainage from small developments.
- Local authority drainage design guides for soakaway testing.