BRE Digest 365 Percolation Test (Vp) Interpreter

After running a BRE Digest 365 percolation test (300×300×300mm test pit, timing the water level fall from 75% to 25% of pit depth), enter your calculated Vp value below to see how your soil classifies for soakaway design.

Percolation Value — Vp (seconds)

Time in seconds for the water level to drop 150mm (the 75%–25% interval) per BRE Digest 365 method.

< 15s — Excellent

15–300s — Suitable

> 300s — Not Suitable

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Method reference: BRE Digest 365, Soakaway Design. This tool classifies a single Vp reading against the two published reference points (Vp<15s = highly permeable / excellent; Vp>300s = generally unsuitable for infiltration). It does not replace a full site investigation, multiple test-pit averaging, or a groundwater-level check — all of which a qualified drainage engineer should confirm before final sizing.