Percolation test calculator: soakaway infiltration rate
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Worked example
| You enter | Value |
|---|---|
| Trial pit length | 1 m |
| Trial pit width | 0.6 m |
| Effective depth | 0.8 m |
| Fills: time for the water to fall from 75% to 25% of the effective depth | Fill 1: time from 75% to 25% 60 min, didn't drain to 25% No |
Infiltration rate f = 3.55 × 10⁻⁵ m/s
| Result | Value |
|---|---|
| Design infiltration rate f (lowest fill) | 3.55 × 10⁻⁵ m/s |
| In mm per hour | 127.7 mm/h |
| Volume from 75% to 25% (V_p75–25) | 0.24 m³ |
| Area to 50% depth, including the base (a_p50) | 1.88 m² |
| Result | Indicative: BRE 365 asks for three fills where possible. |
| Fill | Time 75% to 25% | f |
|---|---|---|
| Fill 1 | 60 min | 3.55 × 10⁻⁵ m/s |
Show the working
- V_p75–25: 1 × 0.6 × (0.75 − 0.25) × 0.8 m = 0.24 m³
- a_p50: 2 × (1 + 0.6) × 0.4 + 1 × 0.6 = 1.88 m²
- Fill 1: 0.24 ÷ (1.88 × 3,600 s) = 3.55 × 10⁻⁵ m/s
- Design f: the lowest fill, 3.55 × 10⁻⁵ m/s (127.7 mm/h)
How it works
- V_p75–25 (m³) = length × width × (75% − 25%) × effective depth
- a_p50 (m²) = 2 × (length + width) × 50% of the effective depth + length × width (sides to half depth, plus the base)
- f (m/s) = V_p75–25 ÷ (a_p50 × t_p75–25), with t in seconds
- Design f = the lowest f of the fills; a pit that doesn't drain means the ground is unsuitable for a soakaway
BRE Digest 365 sets out the trial pit and filling procedure; your building control body may want to see the test.
Take the slowest fill: soil drains more slowly once it's wet.
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
This calculator has no built-in defaults: every figure comes from you or your product's data.
Regulations and standards
- BRE Digest 365 (2016): soil infiltration rate test
- Approved Document H, H3 (rainwater drainage)
Explained: BRE Digest 365 (soakaway design), Approved Document H (drainage).
BRE Digest 365 is used across the UK. Approved Document H covers England; the other nations have their own drainage guidance.