French Drain Calculator — Gravel, Pipe & Membrane

Our French drain calculator works out the gravel volume, pipe length and geotextile membrane area needed for your drain run based on standard trench dimensions.

Gravel calculation

Volume (m³) = Drain length × Trench width × Trench depth

Weight (tonnes) = Volume × density (default 1.5 t/m³; clean 20mm drainage stone typically runs 1.5–1.8 t/m³ — confirm with your supplier).

The volume uses the full trench depth — i.e. a geotextile-wrapped trench filled with gravel up to the surface. If you intend to cap the drain with a layer of topsoil and turf, reduce the gravel depth by the cap thickness before calculating (or simply order less than the figure shown).

Membrane calculation (rule of thumb)

Membrane width = (2 × Depth) + Trench width + 300mm top lap

Membrane area = Membrane width × Drain length

This is a trade estimate, not a British Standard. It allows for lining both sides and the bottom plus a single lap over the top. For a full double fold-over, add an extra lap length, and round the roll quantity up.

Construction method

• Dig trench with minimum 1:100 fall towards outlet
• Line with geotextile membrane (sides and bottom), leaving excess at the top
• Add 50-100mm gravel bed at bottom
• Lay perforated pipe (holes facing down) on gravel bed
• Fill trench with 20mm clean gravel up to the surface
• Fold membrane over the top of the gravel
• Optional: cap with topsoil and turf — reduce the gravel order by the cap depth