Free to size. Building Control-ready PDF £25 +VAT (£30 inc VAT).
Runs in your browser. Enter your percolation results and read off the soakaway size.
The BRE 365 method, done for you
BRE Digest 365 sizes a soakaway by balancing what flows in during a storm against what soaks away through the soil. SoakMonkey takes the soil infiltration rate from your percolation test, the area draining to the soakaway and the design rainfall, then tests a range of storm durations and reports the storage volume — and the physical size — you need.
Rainfall and climate change
The calculation uses design rainfall depths for the site (FSR/FEH) at the chosen return period, with an Environment Agency climate-change uplift applied so the soakaway still works over its design life. You pick the return period and uplift; SoakMonkey does the storm-duration sweep and takes the worst case.
Four construction types
How much water a soakaway can hold depends on what it's built from — an open pit, a granular-filled pit, or modular geocellular crates all store different proportions of their volume. SoakMonkey covers four construction types so the size it gives matches what you'll actually build.
Frequently asked questions
What is BRE Digest 365?
BRE Digest 365 "Soakaway design" is the standard UK method for sizing a soakaway. It uses the soil infiltration rate from a percolation test, the design rainfall for the site, and the soakaway construction to work out the storage volume needed so the soakaway never overflows in the design storm.
What does the BRE 365 calculation need?
The soil infiltration rate (f) from your percolation test, the area draining to the soakaway, the design rainfall depths for a range of storm durations (commonly the 10-year return period, with a climate-change uplift), and the soakaway construction type, which sets how much of its volume is available to store water.
Why check several storm durations?
The worst case is not always the heaviest short storm. BRE 365 checks a range of durations and takes the one that needs the largest storage — a long, moderate storm can fill a soakaway that drains slowly more than a short, intense one. SoakMonkey tests the durations for you.
Is the calculator free?
Yes — entering the percolation results and sizing the soakaway on screen is free. You only pay £25 +VAT to export the Building Control-ready PDF.
Back to the main soakaway calculator, or start with the percolation test calculator.