What size gravel is best for a French drain?

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 (/landscaping/gravel-calculator).

How deep and wide should a French drain be?

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 (/landscaping/drainage-fall-calculator) to work out the correct gradient.

Do I need geotextile membrane in a French drain?

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.

Where does the water from a French drain go?

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 (/landscaping/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.

How do I calculate membrane width for a French drain?

The membrane width needs to line the trench completely with overlap at the top. The formula is: Width = (2 x Trench Depth) + Trench Width + 300mm overlap. For a standard 300mm x 600mm trench: (2 x 600) + 300 + 300 = 1800mm (1.8m) wide membrane. This land drain calculator computes the membrane area for you.

TradeCalcs

French Drain Calculator — Gravel, Pipe & Membrane

Calculate gravel, perforated pipe and geotextile membrane needed for a French drain. Standard UK trench dimensions.

Drain Length

Total length of the French drain

Trench Width

300mm standard width

Trench Depth

600mm standard depth

Include wastage allowance

Adds 10% extra gravel

Wastage Percentage

10% recommended

Price Per Tonne of Gravel (optional)

Enter supplier price for a cost estimate

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 × 1.5 (20mm clean gravel density)

Membrane calculation

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

Membrane area = Membrane width × Drain length

Construction method

Dig trench with minimum 1:100 fall towards outlet
Line with geotextile membrane (sides and bottom)
Add 50-100mm gravel bed at bottom
Lay perforated pipe (holes facing down) on gravel bed
Fill trench with 20mm clean gravel to within 100mm of surface
Fold membrane over the top of the gravel
Backfill with topsoil to ground level

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.