Percolation Test Calculator — Calculate Your Vp Value

Dig a hole 300mm x 300mm square and 300mm deep below the proposed invert level (AD H 2015 §1.34). Fill with water and allow to drain completely, then refill and allow to drain overnight (pre-soaking). The next day, refill to at least 300mm and time the drop from 75% full (225mm) to 25% full (75mm) — a 150mm fall (§1.36). Divide that time by 150 to get Vp. Repeat at least 3 times and use the average.

Vp is the percolation value in seconds per millimetre. It represents the average time for the water level to drop 1mm. Calculated by dividing the average time for a 150mm drop by 150. For example, 3000 seconds average gives Vp = 3000/150 = 20 s/mm.

For a septic-tank drainage field the Vp must be between 12 and 100 seconds per mm. Below 12 means the soil is too permeable (rapid percolation into groundwater); above 100 means too impermeable. This range is per Approved Document H 2015 §1.38. Once you have a valid Vp, use our Soakaway Crate Calculator to size the soakaway based on your percolation results.

The drainage field floor area At = P x Vp x 0.25 m squared, where P is the number of occupants (Approved Document H 2015 §1.44). For 4 people with Vp 30: At = 4 x 30 x 0.25 = 30 m squared. Per §1.42 trenches should be 300–900mm wide (this tool assumes 600mm) on a 300mm layer of clean shingle, with distribution pipes laid at least 500mm below the surface and 2m of undisturbed ground between parallel trenches.

A professional percolation test in the UK typically costs around £100 to £600, with most quotes landing near £300–£500. The price depends on the number of trial holes, site access, soil and groundwater conditions, travel, and whether a full written report sized to Approved Document H 2015 / BS 6297:2007 is included for Building Control. This calculator is free: it works out your Vp value and drainage-field size from your own readings, so you can do the test yourself or sense-check a contractor’s figures. The range above is an indicative UK market estimate (sources: MyBuilder price guide and UK drainage contractors), not a calculated price — always obtain written quotes locally.

You can carry out a percolation test yourself — the Approved Document H 2015 method (§1.34–1.38) only needs a spade or 300mm auger, a tape, water and a stopwatch. Dig a 300mm square hole 300mm below the proposed pipe invert, pre-soak it overnight, then time the 75%-to-25% (150mm) fall and divide by 150 to get Vp; repeat at least three times across at least two trial holes (§1.37). However, Building Control and most installers want a written report, and many homeowners pay a professional for accuracy, a separate trial hole to check the groundwater table (§1.33), and an Approved-Document-H-compliant drainage-field design. Do not test during heavy rain, severe frost or drought (§1.37), as abnormal weather distorts the result.

Approved Document H 2015 §1.27 says a drainage field or mound must be at least 10m from any watercourse or permeable drain, at least 50m from a groundwater abstraction point (and not in a Zone 1 groundwater protection zone), and at least 15m from any building. It should also be far enough from other drainage fields, mounds or soakaways that the ground’s soakage capacity is not exceeded, and should be downslope of groundwater sources (§1.28). No water-supply pipes, other services, access roads, driveways or paved areas should sit within the disposal area (§1.29–1.30).

Yes. To discharge treated effluent to ground under the Environment Agency General Binding Rules (gov.uk, small sewage discharge to the ground), it must pass through a drainage field designed to BS 6297:2007, and Approved Document H 2015 (§1.32–1.38) requires a preliminary site assessment, a trial hole to locate the groundwater table, and a percolation test to size that field. A Vp between 12 and 100 s/mm is needed for a drainage field to be acceptable (§1.38). If your discharge to ground exceeds 2 cubic metres (2,000 litres) per day you must apply to the Environment Agency for a permit rather than rely on the binding rules.