French Drain Calculator
Work out gravel, pipe, fabric and fall for a French drain trench.
Enter your values
Enter the trench length in ft.
Enter the trench width in in.
Enter the trench depth in in.
Enter the pipe diameter in in.
Enter the fall in %.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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
At a glance
| What it does | Work out gravel, pipe, fabric and fall for a French drain trench. |
|---|---|
| Category | Home & DIY |
| Inputs needed | Trench length, Trench width, Trench depth, Pipe diameter, Fall |
| Main output | Gravel needed |
| Formula | gravel yd3 = length x width x depth / 27 - pipe volume |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Trench length(ft)
- Enter the trench length in ft.
- Trench width(in)
- Enter the trench width in in.
- Trench depth(in)
- Enter the trench depth in in.
- Pipe diameter(in)
- Enter the pipe diameter in in.
- Fall(%)
- Enter the fall in %.
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.
What do I need to enter into the French Drain Calculator?
Just 5 values: trench length, trench width, trench depth, pipe diameter, fall. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the French Drain Calculator work out the 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. It applies the formula gravel yd3 = length x width x depth / 27 - pipe volume and shows the working so you can check each step by hand.
Is the French Drain Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.