How to calculate a french drain
A French drain is a gravel filled trench with a perforated pipe that carries water away from a wet area.
- Measure the trench length from the wet spot to the outlet.
- Enter the trench width and the depth of the gravel.
- Subtract the volume of the pipe if you want a tighter number.
- Order clean washed gravel and fabric to wrap the trench.
Worked example
For a typical french drain with length 50 ft, width 12 in, depth 18 in, the volume is 75.00 cubic feet. Divide by 27 for 2.78 cubic yards. With a 10 percent allowance, plan for 3.06 cubic yards.
In gravel, 2.78 cubic yards weighs about 3.89 US tons at a planning value of 2,800 pounds per cubic yard.
French Drain sizes and quantities
These common sizes are calculated with the same formula as the tool above, so you can find your number quickly.
| Size | Cubic feet | Cubic yards | With 10 percent extra | US tons |
|---|---|---|---|---|
| 25 ft, 12 in wide, 18 in deep | 37.50 | 1.39 | 1.53 | 1.94 |
| 50 ft, 12 in wide, 18 in deep | 75.00 | 2.78 | 3.06 | 3.89 |
| 100 ft, 12 in wide, 24 in deep | 200.00 | 7.41 | 8.15 | 10.37 |
| 150 ft, 18 in wide, 24 in deep | 450.00 | 16.67 | 18.33 | 23.33 |
Tips
- A slope of about 1 inch of fall per 8 feet is a common planning figure.
- Washed gravel drains better than gravel with fines.
- Cover the pipe with at least 2 inches of stone on top.
Common mistakes to avoid
- Using gravel with a lot of dust.
- Laying the pipe flat or uphill.
- Leaving out the fabric, which lets silt clog the stone.
These figures are planning estimates. Real material varies in moisture, size, and compaction, so confirm weights and order quantities with your supplier.
Frequently asked questions
How many cubic yards do I need for a french drain?
Multiply the dimensions in feet and divide by 27. The example above, length 50 ft, width 12 in, depth 18 in, needs 2.78 cubic yards before waste.
How much extra should I order?
Add 5 to 10 percent for waste. Add more if the ground is uneven or the material is loose.
Can I enter inches and feet together?
Yes. Each box has its own unit selector, so you can mix feet and inches.
How much does french drain material weigh?
2.78 cubic yards of gravel weighs about 3.89 US tons as a planning value.