Percolation Test Calculator — Calculate Your Vp Value

This calculator processes your percolation test field data to determine the Vp (percolation value) and assess whether the soil is suitable for soakaway or drainage field installation.

The formula

Vp = Average time (seconds) / 150 (mm drop)

The percolation test measures the time (in seconds) for water to drop 150mm in a 300mm × 300mm test hole. Three readings are taken and averaged. The average time is divided by 150 to give the Vp value in seconds per millimetre.

Suitability ranges

• Vp < 15 s/mm: Too permeable — risk of groundwater contamination. Not suitable for soakaway or drainage field.
• 15 ≤ Vp ≤ 100 s/mm: Suitable for soakaway or drainage field. Use the Vp value to size the drainage field.
• Vp > 100 s/mm: Too impermeable — soil cannot absorb effluent quickly enough. Alternative drainage required.

Using the Vp value

Once you have the Vp value, it is used to size the drainage field using the formula:

Drainage field area = P × Vp × 0.25 m²

Where P is the population equivalent (number of people). For example, with Vp = 30 and P = 4: Area = 4 × 30 × 0.25 = 30 m² of drainage field.

Standards

The percolation test procedure and assessment criteria are specified in BRE Digest 365 (Soakaway Design) and BS 6297 (Code of Practice for the Design and Installation of Drainage Fields for Use in Wastewater Treatment). Building Regulations Part H also references these standards for below-ground drainage design.