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French Drain Calculator — Gravel, Pipe & Membrane
Calculate gravel, perforated pipe and geotextile membrane needed for a French drain. Standard UK trench dimensions.
Total length of the French drain
300mm standard width
600mm standard depth
Adds 10% extra gravel
10% recommended
Enter supplier price for a cost estimate
20mm clean drainage stone ~1.5–1.8 t/m³ — confirm with supplier
Standard 850 kg dumpy bag ≈ 0.5 m³
Top lap (rule of thumb): 300mm
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How We Calculate This
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
Frequently Asked Questions
Use clean 20mm gravel (also called 20mm shingle, pea gravel or drainage gravel) for French drains. It provides excellent drainage around the perforated pipe. Avoid using sharp sand or fine aggregates as they will clog the pipe perforations and reduce drainage capacity over time. For decorative or driveway gravel, see our Gravel Calculator.
A standard French drain trench should be 300mm (12 inches) wide and 600mm (24 inches) deep. For areas with heavy waterlogging, increase to 450mm wide and 900mm deep. The trench should have a minimum fall of 1:100 (10mm per metre) towards the outlet or soakaway — use our Drainage Fall Calculator to work out the correct gradient.
Yes — geotextile membrane (also called filter fabric or drainage membrane) is essential. It wraps around the gravel to prevent fine soil particles from migrating into the gravel and clogging the drain. Without membrane, a French drain can become blocked within a few years. Use a non-woven geotextile rated for drainage applications. This French drain material calculator includes membrane area in its output.
French drains should discharge to a soakaway, drainage ditch, or a surface water sewer (with permission from the water company). Size your soakaway using our Soakaway Sizing Calculator. The drain must have a continuous fall towards its outlet. You cannot discharge into a foul sewer. In some cases, Building Regulations approval may be required.
As a trade rule of thumb, the membrane width needs to line both sides and the bottom of the trench plus a lap at the top: Width = (2 x Trench Depth) + Trench Width + 300mm top lap. For a standard 300mm x 600mm trench: (2 x 600) + 300 + 300 = 1800mm (1.8m) wide membrane. This is an estimate, not a British Standard — if you fully fold both sides over the top so they overlap, add an extra fold length, and always round the roll quantity up. This calculator computes the membrane area for you.
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Last updated: February 2026
Verified against UK standards · estimates only, confirm with your supplier.