How do I carry out a percolation test?

Dig a hole 300mm x 300mm square and 300mm deep below the proposed invert level. Fill with water and allow to drain completely. Refill and allow to drain overnight (pre-soaking). The next day, refill to 300mm and time the drop from 300mm to 150mm. Repeat 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.

What Vp value do I need for a soakaway?

The Vp must be between 15 and 100 seconds per mm. Below 15 means soil is too permeable (groundwater contamination risk). Above 100 means too impermeable. This range is per BRE Digest 365 and BS 6297.

How do I size a drainage field from the Vp value?

The drainage field area = P x Vp x 0.25 m squared, where P is the number of occupants. For 4 people with Vp 30: Area = 4 x 30 x 0.25 = 30 m squared. Trenches are typically 600mm wide and 400mm deep, filled with washed gravel.

Percolation Test Calculator — Calculate Your Vp Value

Calculate the percolation rate (Vp value) from your field test readings and determine whether your soil is suitable for a soakaway or drainage field. Based on BRE Digest 365.

Enter the time (in seconds) for water to drop 150mm in your 300mm × 300mm test hole. Take 3 readings after the overnight pre-soak.

Test Reading 1

Time for 150mm water drop

Test Reading 2

Time for 150mm water drop

Test Reading 3

Time for 150mm water drop

Number of Occupants

Population equivalent for drainage field sizing

Soil Type (optional)

Helps interpret your Vp result

How We Calculate This

This calculator processes your percolation test field data to determine the Vp value and size the drainage system.

The formula

Vp = Average time (seconds) / Water drop (mm)

Suitability ranges (BRE DG 365)

Vp < 15 s/mm: Too permeable — groundwater contamination risk
15 ≤ Vp ≤ 100 s/mm: Suitable for soakaway or drainage field
Vp > 100 s/mm: Too impermeable — alternative drainage needed

Drainage field sizing (BS 6297)

Area = P × Vp × 0.25 m² where P = occupants

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.