How this calculator works
A French drain is a trench lined with filter fabric, filled with washed stone around a perforated pipe. The stone is the volume of the trench less the volume of the pipe, plus waste, converted to tons. The fabric wraps the stone with an overlap on top, and the drain needs a slope of at least 1 inch every 8 feet.
Worked example: a 50 ft drain in a trench 12 inches wide and 18 inches deep with a 4 inch pipe takes about 2.9 cubic yards (4.0 tons) of washed stone, 50 ft of pipe (5 lengths), 250 ft² of fabric, and moves about 2.8 cubic yards of soil. It needs 6 inches of fall over its length.
Materials by drain length (12 × 18 in trench, 4 in pipe)
Washed stone with 10% extra.
| Length | Stone | Pipe lengths (10 ft) | Fabric | Fall needed |
|---|---|---|---|---|
| 25 ft | 2.0 tons | 3 | 125 ft² | 3.1 in |
| 50 ft | 4.0 tons | 5 | 250 ft² | 6.3 in |
| 75 ft | 6.0 tons | 8 | 375 ft² | 9.4 in |
| 100 ft | 8.1 tons | 10 | 500 ft² | 12.5 in |
Tips before you buy
- Check the slope with a string line and a level, or a laser. The bottom of the trench must fall toward the outlet the whole way.
- Use washed 3/4 to 1½ inch stone, not stone with fines, which clogs the drain.
- Wrap the stone in non-woven geotextile to keep soil out, but not the pipe alone in fabric sock if the soil is fine and silty: follow the pipe maker.
- Give the water a place to go: daylight at the bottom of a slope, a dry well, or a storm drain where it is allowed.
- Keep the top of the drain a few inches below the surface, and cover it with soil or a layer of stone for a rock-lined swale.
Tools & materials for this project
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Frequently asked questions
How much gravel do I need for a 50 ft French drain?
About 2.9 cubic yards (4.0 tons) of washed stone for a trench 12 inches wide and 18 inches deep, after allowing for the pipe and 10% extra. A trench 18 inches wide and 24 deep takes 8.3 tons.
What slope does a French drain need?
At least 1 inch of fall for every 8 feet (about 1%), all the way to an outlet. Without a slope and an outlet the drain fills with water and does nothing.
Do I need filter fabric around a French drain?
Yes, wrap the stone in non-woven geotextile so soil cannot wash in and clog it. A 50 ft trench takes about 250 ft² of fabric, including the overlap.
Which way do the holes in the pipe face?
Down, in the usual instruction for rigid perforated pipe, so water enters from below and flows along the bottom. Some makers specify otherwise, so check the pipe.
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.