About this tool Calculate the percolation value (Vp) from Building Regulations Part H soakaway test data. Enter the timed readings for each 25mm water level drop — the tool calculates the average Vp and classifies soakaway suitability per Approved Document H.
Vp is the average time in seconds for the water level to drop 1mm. Values between 15 and 100 s/mm indicate the ground is suitable for soakaway drainage. Below 15 is very fast (check contamination risk), above 100 is too slow.
The standard test uses a 300mm square hole filled to 300mm depth after pre-soaking. Minimum three timed 25mm drops. Per Approved Document H (2015), clause 3.30.
How to use this tool 1. Enter the time for each 25mm water level drop — in seconds. Minimum 3 readings required.
2. Add more readings if available — more readings give a more reliable average.
3. Read the Vp and soakaway suitability — 15–100 s/mm is suitable per Approved Document H.
Technical information Vp = (average time for 25mm drop) / 25 — seconds per mm
Infiltration rate (mm/hr) = 3600 / Vp
Test procedure: excavate 300mm square hole to proposed soakaway invert level. Fill three times to pre-soak. Then fill to 300mm and time three consecutive 25mm drops.
Limitations The Vp test is a simplified percolation test for small-scale domestic soakaway design. For larger developments or commercial projects, use BRE 365 falling head tests in trial pits.
Results are point-specific and can vary significantly over short distances. Test at multiple locations representative of the proposed soakaway position.
Revision history 15 September 2026: Initial release
Disclaimer This tool is provided for educational and general information purposes only. It is not a substitute for professional engineering advice, design or verification.
Diggy and its contributors are not licensed engineering consultants and no results generated by this tool should be used directly for construction, design or safety-critical decisions.
All values and outputs are based on published empirical correlations and should be independently checked and confirmed by a qualified geotechnical engineer before use.
By using this tool, you accept full responsibility for how you interpret and apply the information provided.
Spotted an error? Let us know .
