French Drain Calculator
Work out gravel, pipe, fabric and fall for a French drain trench.
Enter your values
ⓘ Live results
Results
- Gravel needed
- 2.62 cubic yards
- Gravel weight
- 3.66 tons
- Perforated pipe
- 50 ft
- Filter fabric
- 250 sq ft
- Total drop over run
- 6 in
Working
- 1trench 2.78 yd3 - pipe 0.16 yd3 = 2.62 yd3 of stone
Quick answer
A French drain needs about 1% fall, gravel to fill the trench around the pipe, and filter fabric wide enough to wrap the stone.
gravel yd3 = length x width x depth / 27 - pipe volume
How to use the French Drain Calculator
- 1Measure the trench run, then choose a width and depth (12 in x 18 in is typical).
- 2Enter the pipe diameter so its volume is subtracted from the stone.
- 3Keep at least 1% fall so water always moves toward the outlet.
Worked example
Using the values the calculator loads with:
Inputs
- Trench length50 ft
- Trench width12 in
- Trench depth18 in
- Pipe diameter4 in
- Fall1 %
Results
- Gravel needed2.62 cubic yards
- Gravel weight3.66 tons
- Perforated pipe50 ft
- Filter fabric250 sq ft
- Total drop over run6 in
- trench 2.78 yd3 - pipe 0.16 yd3 = 2.62 yd3 of stone
Frequently asked questions
How much fall does a French drain need?
A minimum of 1%, or about 1 inch of drop for every 8 feet of run. More fall is fine; less risks standing water.
Do I need filter fabric?
Yes in most soils. Wrapping the stone stops silt migrating in and clogging the voids, which is the usual reason drains fail after a few years.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.