French Drain Calculator
Calculate gravel, pipe, fabric and fall for a French drain trench.
About French Drain Calculator
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.
Formula
French Drain
Gravel volume = trench volume − pipe volume; fall = length × gradient
- geotextile
- Wraps the gravel to stop soil migrating in and clogging it
- fall
- Gradient along the trench, minimum around 1 per cent
Worked example
With the default values below, French Drain Calculator returns Gravel Required of 5.164. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Trench lengthminimum 0 m
- 20 m
- Trench widthminimum 50 mm
- 300 mm
- Trench depthminimum 50 mm
- 600 mm
- Perforated pipe diameterminimum 25 mm
- 100 mm
- Fall along the trenchaccepts 0.1–10 %
- 1 %
Results
- Gravel Required
- 5.164 t
- Gravel Volume
- 3.443 m3
- Geotextile Fabric Area
- 34.50 m2
- Perforated Pipe Length
- 21.00 m
- Total Fall Over the Run
- 200.0 mm
Step by step
- 1Values used
- 2French Drain
- 3Gravel Required= 5.164 t
- 4Gravel Volume= 3.443 m3
- 5Geotextile Fabric Area= 34.50 m2
- 6Perforated Pipe Length= 21.00 m
- 7Total Fall Over the Run= 200.0 mm
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is French Drain calculated?
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.
Why does French Drain matter?
French drains fail from silt clogging far more often than from undersizing, which makes the geotextile wrap the component that determines service life.
What values do I need to enter?
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.