What a 20 ft French drain takes
| Item | Amount |
|---|---|
| Drain rock (clean washed stone, 3/4 to 1½ in) | 1.15 cubic yards, 1.6 tons |
| Perforated pipe, 4 in | 20 ft (2 pieces of 10 ft) |
| Non-woven filter fabric | 100 sq ft |
| Soil to dig out | 1.1 cubic yards |
For a trench 12 in wide and 18 in deep, with 10% extra stone for uneven trench walls and settling. The stone is the trench volume less the space the pipe takes up, at 1.4 tons per cubic yard.
20 ft of drain, by trench size
Width × depth of the trench, with a 4 in pipe and 10% extra stone.
| Trench | Drain rock, cubic yards | Tons | Fabric, sq ft | Soil dug out, cubic yards |
|---|---|---|---|---|
| 8 × 12 in | 0.47 | 0.7 | 73 | 0.5 |
| 12 × 12 in | 0.74 | 1.0 | 80 | 0.7 |
| 12 × 18 in | 1.15 | 1.6 | 100 | 1.1 |
| 12 × 24 in | 1.56 | 2.2 | 120 | 1.5 |
| 18 × 24 in | 2.37 | 3.3 | 130 | 2.2 |
The fall over 20 feet
A French drain only works if the pipe runs downhill. At the usual minimum of 1 inch of fall for every 8 feet, a 20 ft drain drops 2.5 inches from the high end to the outlet. On level ground that means the trench is 2.5 inches deeper at the outlet end than where it starts.
The stone order hardly changes with the slope: it is still about 1.2 cubic yards (1.6 tons) for a 12 by 18 in trench. Set the fall with a string line and a level before you tip any stone in. See slope and grade for drainage.
Use clean, washed stone with no fines, wrap the stone (not the pipe) in the fabric, and make sure the drain has an outlet. The fittings, tools and steps are in the French drain materials list, and the method behind the stone figure is in how much gravel for a French drain.
Want to change anything else? The calculator above is the full French Drain Calculator: edit any number and the results update instantly.
Tools & materials for this project
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Frequently asked questions
How much gravel do I need for a 20 foot French drain?
About 1.2 cubic yards, or 1.6 tons, of clean drain rock for a trench 12 in wide and 18 in deep with a 4 in pipe, including 10% extra. A 12 by 24 in trench takes 1.6 cubic yards.
How much slope does a 20 ft French drain need?
At least 1 inch for every 8 feet, so about 2.5 inches of fall over 20 ft.
How many tons of rock for a 20 ft French drain?
About 1.6 tons for a 12 by 18 in trench, or 2.2 tons if the trench is 24 in deep.
Related sizes and guides
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.