French Drain Calculator
The materials for a French drain — the gravel in cubic metres and tonnes after the pipe's volume is taken out, the perforated pipe, the filter fabric to line the trench, and the spoil to remove — from the trench length, width and depth.
A French drain is a gravel-filled trench with a perforated pipe along the bottom, wrapped in filter fabric so soil cannot clog it.
How the french drain calculator works
A French drain is a gravel-filled trench with a perforated pipe along the bottom, wrapped in filter fabric so soil cannot clog it. The trench volume is length by width by depth; subtract the pipe and what remains is gravel, which weighs about 1.6 tonnes a cubic metre. The fabric lines the base and both sides and folds over the top, so it needs the trench perimeter plus overlap along the whole length. Everything dug out is spoil to take away.
Formula: trench = L × W × D; gravel = trench − pipe volume; fabric = L × (W + 2D + overlap)
Worked examples
| Inputs | Gravel needed | Note |
|---|---|---|
| A 15 m trench, 300 × 450 mm | 1.91 m³ (3.05 t) | about 3.2 tonnes of gravel |
| A deeper trench | — | a third more |
| A wider pipe | — | a little less gravel |
FAQFrequently asked questions
How big should a French drain trench be?
About 300 mm wide and 450 to 600 mm deep for a garden drain, with the pipe at the bottom on a fall of at least 1 in 100 toward the outlet. Deeper for foundation drains.
What gravel should I use?
Clean, washed 20 mm aggregate — no fines, which clog. It weighs about 1.6 tonnes per cubic metre loose, and a trench takes more than people expect.
Why the fabric?
Geotextile keeps soil out of the gravel so the drain keeps flowing for decades. Line the trench, lay the gravel and pipe, and fold the fabric over the top before backfilling.
Perforated pipe or none?
A pipe moves water far faster than gravel alone and is worth including on anything over a few metres. Holes face down on the outlet side of the fall; wrap the pipe in a sock in silty soils.
What do I do with the spoil?
It is the full trench volume and it will not go back in — the gravel has taken its place. Plan a skip or a use for it.
Where these figures come from
- The Engineering ToolBox — material densities, coverage and unit data — bulk densities, concrete at 2,400 kg/m³, board and sheet sizes
- Brick Development Association — brick sizes and quantities — 215 × 102.5 × 65 mm, 60 bricks per m²
- UK Government — Approved Documents (Building Regulations) — Building Regulations guidance
Last checked: September 2026. Coverage rates and material sizes are typical manufacturer figures — check the product you buy.