A French drain is a trench of clean stone that gives water somewhere easier to go than through your wall. Almost every one that fails does so for the same two reasons: the wrong stone, or nowhere for the water to come out.
Enter the trench as length × width, then the depth of stone. Most domestic drains are 300 mm wide and 400–600 mm deep.
Works in metric and US imperial — you choose at the start and can switch at any time. Every figure this calculator uses is published with its source, and the result comes with a printable order sheet. No account, no email, and nothing you type leaves your browser.
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Trench length × width × the depth of stone, plus a settlement allowance. Most domestic drains are 300 mm wide and 400–600 mm deep. A 10 m run at 300 mm wide and 450 mm of stone is 1.35 m³ in place — about 1.55 m³ to order, roughly 2.4 tonnes of clean stone.
The wrong stone, or nowhere for the water to go. Clean angular stone is the right material — rounded pea gravel drains well but does not lock up, so it is fine in a wrapped trench and poor anywhere it has to hold a shape or take load. And check where the water ends up before you dig: a drain discharging onto a neighbour's land, or into a soakaway too small for the volume, is a problem moved rather than solved.
Wrap the stone completely in geotextile — sides and top, generous overlap. It is the highest-value item on the order and the one most often left out; without it fine soil migrates in, fills the voids, and a silted drain cannot be cleaned, only dug up. Perforations face down, because water enters the pipe from below through the stone; holes up means the trench has to fill before the pipe does anything. And plan for the spoil: the trench is now full of stone, so the soil you dug out has nowhere to go — and it has swelled about 25% in the digging.
Most domestic drains are 300 mm wide and 400–600 mm deep. Deeper is not automatically better: what matters is that the stone is wrapped, the fall is continuous, and the water has somewhere legitimate to go at the end.
Down. Water enters the pipe from below through the stone. With the holes facing up, the trench has to fill before the pipe does anything at all.
Yes — sides and top, with a generous overlap. The stone works because its voids stay open, and fine soil migrating in from the sides fills them. A drain that has silted up cannot be cleaned out; it has to be dug up and rebuilt.
Be careful. A trench alongside a wall can undermine a footing, and a drain keeping the ground next to a foundation permanently wet is not an improvement. Anything close to a foundation is worth a professional opinion rather than a calculator.