Free to calculate. Building Control-ready PDF £25 +VAT (£30 inc VAT).
Work out your infiltration rate
Your trial pit and its fall times. It all works in this page — nothing is sent anywhere.
Time for the water to fall from 75% to 25% of the effective depth, for each fill:
Enter the pit size and at least one fall time.
f = V(75–25) ÷ (a(p50) × t), where V = L × W × d ÷ 2 is the water drained between the 75% and 25% levels and a(p50) = (L + W) × d + L × W is the pit’s sides up to half depth plus its base. The lowest fill is the design rate. New to the test? How to do a BRE 365 soakaway test, step by step.
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 quickly to its effective depth — a bowser is the usual way. 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%, and note the level against time as it goes. SoakMonkey's stopwatch does this on your phone and writes the time into the test. If the level never reaches 25%, the fill has not given a rate — don’t extrapolate it.
- 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.
- Write it up. Date, pit position, pit size and depth, what it was dug in, whether groundwater showed, and each fill. SoakMonkey’s free test record PDF lays all of that out from what you type in.
The full method — kit, the water you will need, when to test, and the mistakes that get a test sent back — is in how to do a BRE 365 soakaway test.
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. Approved Document H also accepts that small-hole test for a small soakaway, and gives a formula to convert Vp to an infiltration rate. But where the request is for a soakaway designed to BRE 365 — as planning drainage conditions and lead local flood authorities usually put it — the trial-pit method above is the test, and it is the one SoakMonkey takes. Ask your building control body which it accepts before you dig.
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 quickly to its effective depth, and time how long the level takes to fall from 75% to 25% of that depth. 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, stamped with the date and time. The free test record PDF then lays out the whole test — pit, fills, rate, groundwater — for Building Control.
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 including the base, 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. Approved Document H does let the small-hole test stand in for a small soakaway, with a formula to convert Vp to an infiltration rate, but a BRE 365 design needs the trial pit — and the trial pit is the test SoakMonkey takes.
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, read the step-by-step BRE 365 soakaway test, or the BRE Digest 365 method.