Free to calculate. Building Control-ready PDF £25 +VAT (£30 inc VAT).
Runs in your browser. Enter your three test runs and read off Vp.
What the percolation test measures
The percolation test tells you how fast water soaks into the ground — the single most important number for sizing a soakaway. You time how long the water level takes to fall over a 150 mm test depth, repeat it across three runs and at least two trial holes, and average the results. SoakMonkey turns those timings into the percolation value Vp (seconds per millimetre).
From Vp to a soakaway size
Vp feeds straight into the BRE Digest 365 calculation: a lower Vp means faster-draining soil and a smaller soakaway, a higher Vp means a bigger one. SoakMonkey carries your Vp through automatically and sizes the soakaway against the design storm, 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 Vp leads to an impractical size, so you can consider an alternative outfall early rather than discovering it on site.
Frequently asked questions
How do you carry out a percolation test?
Dig a trial pit, pre-soak it, then fill the test section with water and time how long the level takes to fall from 75% to 25% of the effective depth (a 150 mm fall). Repeat over three runs across at least two trial holes, and use the average — soil and weather vary, so a single reading is not enough.
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 percolation rate (Vp) calculated?
The percolation value Vp is the average time in seconds for the water to fall over the 150 mm test depth, divided by 150 — giving a rate in seconds per millimetre. A lower Vp means faster-draining soil. SoakMonkey works Vp out from your timings and carries it straight into the BRE 365 soakaway sizing.
What Vp is too slow for a soakaway?
Very slow soils make a soakaway impractical — the water cannot disperse fast enough between storms. The calculator flags when your Vp gives a soakaway size that is unworkable, so you find out before you dig, not after.
Is it free?
Yes — entering your test timings and getting Vp 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.