How to measure for your french drain gravel project
- Measure and dig your trench. Mark the trench route from the wet area to the outlet point (a daylight end on a slope, a dry well, or a storm drain — never the sanitary sewer). Dig 12–18 inches wide and 18–24 inches deep, sloping toward the outlet at roughly 1% grade — about 1 inch of drop for every 8–10 feet of run.
- Enter dimensions and read your materials list. Type in the trench length, width, and depth, then choose your pipe diameter (4 inches is standard for residential drains). The calculator subtracts the pipe volume from the trench volume so you get the exact gravel needed, plus the filter fabric area and pipe length.
- Install in layers with fabric. Lay filter fabric in the trench with plenty of overlap up the sides. Add a few inches of gravel, lay the perforated pipe (holes down so water enters and exits through the gravel bed rather than collecting debris from above), fill with gravel to a few inches below grade, then fold the fabric over the top. Cap with topsoil or sod.
How the french drain gravel calculator works
Gravel fills the trench volume minus the space taken up by the pipe. The trench volume is length × (width ÷ 12) × (depth ÷ 12) in cubic feet. The pipe displaces a cylinder: π × (diameter ÷ 24)² × length. Subtract the pipe volume and divide by 27 to get cubic yards. For a 50-ft trench 12 in wide × 18 in deep with a 4-in pipe: trench = 50 × 1 × 1.5 = 75 cu ft; pipe = 3.14159 × (0.1667)² × 50 ≈ 4.36 cu ft; gravel = 75 − 4.36 = 70.64 cu ft ÷ 27 = 2.62 cubic yards. At roughly 1.4 tons per cubic yard, that is about 3.66 tons. Filter fabric is sized at the trench wetted perimeter (bottom + two sides) plus 20% for overlap: 50 × ((12 + 36) ÷ 12) × 1.2 = 240 sq ft.
Which type are you estimating?
Standard French drain (perimeter or yard)
The classic layout: a gravel-filled trench with a perforated pipe that collects groundwater and carries it to a daylight outlet or dry well. Best for a low spot in a yard or water seeping across a flat area. Slope the trench at least 1% (1 in per 8–10 ft) toward the outlet so water moves by gravity.
Enter: Enter trench length, 12 in wide × 18 in deep, 4 in pipe
Curtain drain (interceptor across a slope)
Runs horizontally across a hillside or slope to intercept surface and subsurface water before it reaches a structure, lawn, or garden bed. Typically shallower than a standard drain — often 12–18 in deep — and routed to daylight on the downhill side. Use the same calculator; just enter the width of the slope you are intercepting as the trench length.
Enter: Enter slope width as trench length, 12 in wide × 12–18 in deep, 4 in pipe
Footing or foundation drain
Runs along the base of a foundation wall to keep hydrostatic pressure from pushing water through basement walls or a crawl space. The trench bottom should sit at or below the footing elevation. A wider, deeper trench (18 in wide × 24 in deep is common) holds more gravel and handles larger water volumes. Water is routed to a sump pit or a daylight outlet away from the house.
Enter: Enter foundation perimeter as length, 18 in wide × 24 in deep, 4 in pipe
Rigid perforated PVC pipe
Rigid Schedule 20 or SDR-35 perforated PVC is cleanable with a drain snake, less likely to crush under heavy soil or vehicle loads, and can last decades. It costs more than flex pipe but is the better long-term choice when the trench is straight and access allows. Connections are made with standard PVC couplings.
Enter: Standard 4 in diameter; use same trench dimensions as above
Corrugated flexible (flex) pipe
Corrugated HDPE flex pipe is inexpensive, bends around obstacles without fittings, and is sold in coils at every home center. The corrugations can collect sediment over time and it cannot be cleaned as easily as PVC. Best for straight short runs or where the drain path has curves. Use the same pipe diameter in the calculator — the gravel estimate is identical.
Enter: Standard 4 in diameter; trench and gravel figures unchanged
Tips & ways to save
- Slope the trench bottom at least 1% toward the outlet — roughly 1 inch of drop for every 8 feet of run. A string line and a line level make this easy to check as you dig.
- Install the perforated pipe with the holes facing down. Water rises up through the gravel bed and enters the pipe from below; holes-up lets debris fall straight in and clogs the pipe faster.
- Use #57 washed crushed stone (3/4-inch). Fine gravel, pea gravel, or anything with fines will migrate into the pipe over time and eventually block drainage.
- Wrap all the gravel — not just the pipe — in filter fabric. Fold the fabric edges over the top before backfilling so soil cannot work its way down through the stone from above.
- Drain water must go somewhere legal: a daylight end on a slope, a dry well, or a storm drain. Never connect to the sanitary sewer and never outlet onto a neighbor's property.
Gravel for a french drain by trench size (4 in pipe)
| Trench (L × W × D) | Gravel (cu yd) | Gravel (tons) |
|---|---|---|
| 25 ft × 12 in × 18 in | 1.31 | 1.83 |
| 50 ft × 12 in × 18 in | 2.62 | 3.66 |
| 50 ft × 12 in × 24 in | 3.54 | 4.96 |
| 100 ft × 12 in × 18 in | 5.23 | 7.33 |
| 100 ft × 18 in × 24 in | 10.79 | 15.10 |
Gravel volume already subtracts a 4-inch perforated pipe at ~1.4 tons per cubic yard. Use clean, washed ¾-inch stone, and line the trench with filter fabric.
Frequently asked questions
How much gravel do I need for a french drain?
What kind of gravel goes in a french drain?
Do I need landscape fabric in a french drain?
How deep should a french drain be?
Should the perforated pipe holes face up or down?
Where does the water from a french drain go?
Sources
Related calculators
Reviewed by the BackyardCalc editorial team. Figures are computed from the formula above and checked against manufacturer yields.