Percolation Test Calculator — Calculate Your Vp Value

A percolation test measures how quickly water soaks away through your subsoil, expressed as the percolation value Vp (seconds per millimetre). It is the standard UK test for deciding whether a site can take a septic-tank or treatment-plant drainage field, and for sizing that field. This calculator turns your field readings into a Vp value and a drainage-field size to Approved Document H 2015 and BS 6297:2007. Enter the timings yourself or use it to sense-check a contractor’s figures and quote.

Before you test: trial hole & groundwater (AD H 2015 §1.32–1.33)

First carry out a preliminary site assessment (including consulting the Environment Agency and local authority) and dig a trial hole — at least 1 m² and 2 m deep, or 1.5 m below the proposed pipe invert — to find the standing groundwater table. The water table must not rise to within 1 m of the invert of the effluent distribution pipes. Allow for higher winter groundwater if you test in summer.

The percolation test method (AD H 2015 §1.34–1.37)

• Excavate a 300 mm square hole to 300 mm below the proposed pipe invert (or use a 300 mm auger for deep work).
• Fill to at least 300 mm deep and let it drain away overnight (the pre-soak).
• Next day, refill to at least 300 mm and time, in seconds, the fall from 75% to 25% full — a 150 mm drop.
• Repeat at least three times across at least two trial holes and average the results.
• Do not test in abnormal weather (heavy rain, severe frost or drought), as this distorts the reading.

The formula

Vp = Average time (seconds) / Water drop (mm) — with the standard 150 mm drop this is simply the average time divided by 150.

Suitability ranges (AD H 2015 §1.38)

• Vp < 12 s/mm: Too permeable — rapid percolation / groundwater contamination risk
• 12 ≤ Vp ≤ 100 s/mm: Suitable for a drainage field
• Vp > 100 s/mm: Too impermeable — alternative drainage needed

Drainage field sizing (AD H 2015 §1.44)

Area = P × Vp × 0.25 m² where P = number of persons served. This tool then estimates trench length assuming 600 mm-wide trenches (AD H §1.42 permits 300–900 mm).

Siting & the rules (AD H §1.27, Environment Agency)

A drainage field must be at least 10 m from any watercourse or permeable drain, 50 m from a groundwater abstraction point, and 15 m from any building. Under the gov.uk General Binding Rules, treated sewage discharged to ground (up to 2 m³/day) must pass through a drainage field designed to BS 6297:2007; above that volume you need an Environment Agency permit.