Calculate gravel, pipe, fabric and fall for a French drain trench.
The pipe displaces gravel, so its volume is deducted. Fabric area covers the trench base and both sides plus an overlap allowance, and it is what determines whether the drain keeps working after a few years. French drains fail from silt clogging far more often than from undersizing, which makes the geotextile wrap the component that determines service life.
French Drain
Gravel volume = trench volume − pipe volume; fall = length × gradient
Gravel volume = trench volume − pipe volume; fall = length × gradient The pipe displaces gravel, so its volume is deducted. Fabric area covers the trench base and both sides plus an overlap allowance, and it is what determines whether the drain keeps working after a few years.
French drains fail from silt clogging far more often than from undersizing, which makes the geotextile wrap the component that determines service life.
This calculator takes 5 inputs: Trench length, Trench width, Trench depth, Perforated pipe diameter, Fall along the trench. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.