Rented trenchers, delivery fees, an outlet or dry well, and repairing the lawn are not included.
How the French drain calculator works
FrenchDrainCalc treats the drain as a rectangular trench. With a slope, the bottom drops by length × slope while the top stays at ground level, so the average depth is the high-end depth plus half the fall. Gravel = trench volume − pipe volume (π × radius² × length), converted to cubic yards and to tons at 1.4 tons per cubic yard of washed drain rock, or to 0.5 cu ft bags. Filter fabric that wraps the whole trench needs a strip as wide as both walls plus the bottom and a 12 in overlap on top: 2 × depth + 2 × width + 12 in. Pipe is counted in 10 ft sticks or 100 ft coils, and the excavated soil swells about 25% once dug.
Gravel for 10 ft of level trench, 4 in pipe
| Trench | Gravel | Tons | Fabric width |
|---|---|---|---|
| 6 × 12 in | 4.13 cu ft (0.153 cu yd) | 0.214 | 4 ft |
| 8 × 18 in | 9.13 cu ft (0.338 cu yd) | 0.473 | 5.33 ft |
| 12 × 18 in | 14.13 cu ft (0.523 cu yd) | 0.733 | 6 ft |
| 12 × 24 in | 19.13 cu ft (0.708 cu yd) | 0.992 | 7 ft |
| 18 × 24 in | 29.13 cu ft (1.079 cu yd) | 1.51 | 8 ft |
| 24 × 36 in | 59.13 cu ft (2.19 cu yd) | 3.066 | 11 ft |
Gravel density varies with stone size and moisture (about 1.3 to 1.5 tons per cubic yard), so ask your supplier. Call 811 before digging to have buried utilities marked, keep the outlet lower than every point of the drain, and check local rules before sending water toward a neighbour or the street. Prices are your own estimates; results are for planning.
Frequently asked questions
How much does a French drain cost per foot?
Materials often run $4–10 per foot for a typical yard drain; installed prices are usually much higher because of the digging.
What makes a French drain expensive?
Length and depth (more digging and gravel), hard or rocky soil, access for machines, and restoring landscaping.
Is it cheaper to buy gravel in bulk?
Usually, once you need more than about a cubic yard; bags cost more per ton but avoid a delivery fee.
Is FrenchDrainCalc free?
Yes, with no sign-up or ads.
Are my numbers saved?
Yes, in this browser only.