Calculators/ Landscaping/French Drain Calculator
French Drain Calculator
Gravel, pipe and fabric for a drainage trench.
French Drain Calculator
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A French drain works because water takes the easiest path, and clean stone around a perforated pipe is easier than soil. The two things that make it fail are too little fall and no filter fabric, which lets silt clog the stone within a couple of seasons.
How this is calculated
- Trench volume = length × width × depth. Subtract the volume the pipe itself occupies to get the gravel needed.
- Filter fabric has to line the trench and overlap at the top, so its width is the trench width plus both sides plus a foot of lap.
- Aim for at least a 1% fall, which is an inch of drop for every eight feet of run.
Frequently asked questions
Do I need filter fabric?
Yes. Without it, silt migrates into the stone and the drain clogs, often within two or three years. It is the cheapest part of the job and the one most often skipped.
Should the pipe holes face up or down?
Down. Water rises into the pipe from the stone bed below, and holes facing down let the pipe drain rather than hold water.