Free to calculate. Building Control-ready PDF £25 +VAT (£30 inc VAT).
Runs in your browser. Enter the pit size and your three fall times and read off the infiltration rate.
What the percolation test measures
A soakaway only works if the ground around it can take the water away between storms, so the first thing BRE Digest 365 asks for is a measured soil infiltration rate — how fast water soaks out of a hole in the actual ground, at the actual depth, on the actual site. Everything else in the design (the storage volume, the physical size, the half-drain check) is built on that one number. A published soil type is not a substitute.
How to do the BRE Digest 365 percolation test
- Dig a trial pit where the soakaway will go, to the depth it will actually reach — 0.3 to 1 m wide and 1 to 3 m long. Measure and record the length, width and depth; the calculation needs the pit geometry, not just the times.
- Fill it with water to at least 75% of its effective depth. Effective depth is measured from the base to the invert of the inlet pipe.
- Time the fall from 75% to 25%. Start the clock as the level passes 75%, stop it at 25%. SoakMonkey's stopwatch does this on your phone and writes the time into the test.
- Repeat three times, on the same day or on consecutive days, so the surrounding soil is saturated. Dry ground drinks the first fill and flatters the result.
- Design on the slowest fill. The rate is f = V(75–25) ÷ (a(p50) × t(75–25)) in m/s; the calculator works it out from the pit size and each time, and adopts the lowest.
From infiltration rate to soakaway size
The rate feeds straight into the BRE Digest 365 calculation: a faster-draining soil needs a smaller soakaway, a slower one a bigger soakaway. SoakMonkey carries your rate through automatically, sizes the soakaway against the 1-in-10-year design storm across the full range of durations, applies the climate-change uplift and runs the half-drain check — so you go from raw timings to a Building Control-ready answer in one place.
It flags unworkable ground
If your soil drains too slowly for a soakaway to work, it's far cheaper to know before you dig. The calculator flags when the infiltration rate leads to an impractical size or a failed half-drain check, so you can consider an alternative outfall early rather than discovering it on site.
Not the same as the drainage-field (Vp) test
"Percolation test" also means the test in Approved Document H for a septic-tank drainage field: a 300 mm square hole, a 150 mm fall timed in seconds and divided by 150 to give a percolation value Vp in seconds per millimetre, averaged over several runs, with a field only viable between Vp 12 and 100. That is a different test with a different formula for a different structure. A soakaway for surface water uses the BRE 365 trial-pit method above, and that is the one SoakMonkey does.
Frequently asked questions
How do you carry out a percolation test for a soakaway?
The BRE Digest 365 way: dig a trial pit representative of the soakaway — 0.3 to 1 m wide and 1 to 3 m long, dug to the depth the soakaway will actually reach — fill it with water to at least 75% of its effective depth, and time how long the level takes to fall from 75% to 25%. Repeat the fill three times, on the same day or consecutive days, so the surrounding ground is saturated rather than dry, and design on the slowest of the three. Record the pit dimensions: the calculation needs them as much as the times.
Can SoakMonkey time the percolation test for me?
Yes — open the soakaway calculator on your phone at the trial pit (it installs like an app and works offline) and use the built-in stopwatch: start it as the water passes 75% effective depth, stop at 25%, and the time is written straight into the test row.
How is the soil infiltration rate calculated?
From the trial pit geometry and the fall time: f = V(75–25) ÷ (a(p50) × t(75–25)), where V is the volume of water drained between the 75% and 25% levels, a(p50) is the internal surface area of the pit up to 50% of its effective depth, and t is the time taken. The result is in metres per second — soakaways typically work in ground between about 1×10⁻⁶ and 1×10⁻³ m/s. SoakMonkey does the arithmetic from your pit size and three timings and adopts the lowest rate.
What infiltration rate is too slow for a soakaway?
Below roughly 1×10⁻⁶ m/s — heavy clay, typically — a soakaway cannot disperse the water between storms and the BRE 365 half-drain check fails at any sensible size. The calculator tells you when your test results lead to an impractical soakaway, so you can look at an alternative outfall before anything is dug.
Is this the same as the percolation test for a septic tank drainage field?
No. The drainage-field test in Approved Document H (and BS 6297) uses a 300 mm square hole, times a 150 mm fall in seconds and divides by 150 to give a percolation value Vp in seconds per millimetre, averaged over several runs — a drainage field needs a Vp between 12 and 100. The soakaway test is the BRE Digest 365 trial-pit method above, which gives an infiltration rate in m/s. They answer different questions and are not interchangeable; SoakMonkey does the soakaway one.
Is it free?
Yes — entering your pit dimensions and timings and getting the infiltration rate and soakaway size is free in your browser. You only pay £25 +VAT for the Building Control-ready soakaway report.
Back to the main soakaway calculator, or read about the BRE Digest 365 method.