What is a drainage field?

A drainage field (also called a soakaway field or leach field) disperses treated effluent from a septic tank or sewage treatment plant into the ground via perforated pipes laid in gravel-filled trenches. The soil provides final treatment by filtering bacteria. Design follows BS 6297 and Building Regulations Part H.

How do I calculate drainage field pipe length?

Pipe length is calculated from the Vp percolation rate (from a percolation test) and the daily effluent volume. The formula is: Pipe Length = Daily Flow / Vp rate. A typical 4-person household produces about 600 litres/day, and Vp rates of 12-100 sec give acceptable percolation.

What is the maximum trench length?

Individual distribution trenches should not exceed 18 metres in length to ensure even distribution. If more pipe length is needed, use multiple parallel trenches connected via a distribution chamber. Trenches must be at least 2 metres apart (centre to centre).

Do I need vent pipes?

Yes. Each trench should have a vent pipe (typically 110mm) at the downstream end, extending at least 1m above ground level with a mesh cap. Vent pipes allow air circulation through the drainage field, which is essential for the aerobic treatment process in the soil.

What gravel do I need for a drainage field?

The trenches are filled with 20-50mm clean gravel to a depth of at least 300mm below the pipe and 50mm above. The pipe sits in the middle of the gravel bed. A geotextile membrane is laid over the gravel before backfilling with topsoil to prevent silt migration into the gravel.

TradeCalculator

Perforated Pipe Calculator — Drainage Field Length & Trench Layout

Calculate drainage field perforated pipe length from Vp percolation rate, trench spacing, distribution chamber and vent pipe requirements.

Vp Percolation Rate

From percolation test (leave 0 for default)

Occupants

150 L/person/day flow

Daily Flow (optional)

Override auto-calculation

Price Per Metre (optional)

Installed cost per metre

How We Calculate This

This calculator determines the perforated pipe length and trench layout for a drainage field based on percolation test results.

The formula

Pipe Length = Daily Flow (L) / Vp Rate (L/m)

Number of Trenches = ceil(Pipe Length / 18m max)

Gravel Volume = Trenches × Length × Width × 0.3m

Key requirements (BS 6297)

Trench spacing: 2m centre-to-centre minimum
Max trench length: 18m
Pipe gradient: 1:200 along trench
Min depth: 500mm below ground

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.