French Drain Gravel Calculator — How Much Gravel for a French Drain?
Enter your trench dimensions to get the gravel volume for a French drain. No sign-up, no data stored.
Gravel needed
How French drain gravel is estimated
- Trench volume (cu ft) = length x width/12 x depth/12.
- Pipe displacement = π x (d/24)² x length for a d-in pipe (assumes gravel surrounds the pipe).
- Gravel volume = trench - pipe displacement.
- Cubic yards = cu ft / 27; tons = yd x 1.4.
That pipe allowance matters - a 4-in pipe in a 12-in trench is about 9% of the volume. The " "trench cross-section assumes straight walls; if you dig with sloping sides (typical in soft soil), " "add 10-15% to the order.
Drain length → gravel reference
12-in wide, 18-in deep, 4-in pipe, 1.4 tons/yd.
| Length | Trench vol | Gravel vol | Cu yd | Tons |
|---|---|---|---|---|
| 25 ft | 37.5 | 35.3 | 1.3 | 1.8 |
| 50 ft | 75 | 70.6 | 2.6 | 3.7 |
| 75 ft | 112.5 | 105.9 | 3.9 | 5.5 |
| 100 ft | 150 | 141.2 | 5.2 | 7.3 |
Last updated: October 2026 · Order 10-15% extra for sloped trench walls and settling. Confirm the stone density with your supplier.
French drain FAQ
If water pools against the house, regrade the soil first - 5% slope away from the foundation " "solves many wet spots. A French drain is for water that collects in a low area or along a " "foundation where regrading can't reach.
1/8 in per foot toward a daylight outlet, dry well, or storm connection - never a septic " "field. On a 50-ft run that's 6 in of total drop. Check the outlet before digging; a drain " "without an exit is a pond you built yourself.
Crushed limestone packs tight and can trap fines - fine for a base layer, poor for the pipe " "zone. Use washed 3/4-in crushed stone (often called drain rock) for the bed around the pipe " "and clean gravel above it.
Properly built with fabric and washed stone, 20-40 years. The failure mode is silting - soil " "washing into the gravel. Good fabric, a pipe sock, and keeping surface runoff away from the " "trench are what extend the life.
Leave gravel visible only if you like the look; most people cover the top 2-3 in with soil " "and sod so the drain disappears. Keep a marker where the cleanout sits so you can flush the " "line later.
Related calculators & guides
How Much Gravel for a French Drain?
Trench sizing, stone types and fabric for drains.
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