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.
From percolation test per BS 6297 (seconds for water to fall 1mm). Typical 12-100. Leave 0 for default 20.
150 L/person/day flow
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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 (BS 6297:2007)
Area A (m²) = p × Vp × 0.25
Pipe Length = A / Trench Width
Number of Trenches = ceil(Pipe Length / 18m max)
Gravel Volume = Trenches × Length × Width × 0.3m
where p is population equivalent and Vp is percolation value in seconds/mm from the test.
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.