How the calculator handles a trench
A trench is a long, narrow box, so the gravel volume is:
Cubic yards = length (ft) × width (ft) × depth (ft) ÷ 27
Enter width in feet (12 inches = 1, 8 inches = 0.67, 6 inches = 0.5) and depth in inches. The calculator converts the depth for you.
Example. 50 ft long, 1 ft wide, 18 inches deep:
- Volume: 50 × 1 × 1.5 = 75 cu ft = 2.78 cubic yards
- Less a 4 inch pipe: 50 × 0.087 = 4.4 cu ft, leaving about 70.6 cu ft (2.6 cubic yards)
- Weight in #57 stone at about 1.35 tons per yard: about 3.5 to 3.75 tons
If the top few inches of the trench will be soil and sod instead of stone, subtract that depth before you calculate.
Gravel per 10 feet of trench
| Trench (width × depth) | Cubic feet | Cubic yards | Tons of #57 (about) |
|---|---|---|---|
| 6 × 18 in | 7.5 | 0.28 | 0.38 |
| 8 × 18 in | 10.0 | 0.37 | 0.50 |
| 12 × 18 in | 15.0 | 0.56 | 0.75 |
| 12 × 24 in | 20.0 | 0.74 | 1.00 |
Multiply by your trench length ÷ 10. A 70 ft run of a 12 × 18 in trench is 7 × 0.56 = 3.9 cubic yards.
Building a French drain, layer by layer
- Dig to depth, sloping the bottom toward the outlet (a drop of about 1 inch every 8 feet is a common minimum).
- Line the trench with non-woven landscape filter fabric, leaving enough at the sides to fold over the top.
- Bed with 2 to 3 inches of clean stone.
- Lay the pipe. Perforated pipe is usually laid with the holes facing down, so water rises into it from below.
- Cover the pipe with stone to a few inches from the top.
- Fold the fabric over the stone, then finish with stone, or with soil and sod.
Which stone to buy
| Stone | Good for a French drain? |
|---|---|
| #57 / 3/4 in clean stone | Yes, the usual choice |
| 1–1.5 in washed stone | Yes, drains even faster |
| River rock | Yes, if washed and 3/4 in or larger |
| Pea gravel | Works, but packs tighter and can carry fines |
| Crusher run / 3/4 minus | No, the fines clog the drain |
If the drain protects a foundation or handles a lot of runoff, have a drainage contractor or engineer size it. The calculator only estimates stone.
Questions people ask
- How much gravel do I need for a French drain?
- Multiply trench length × width × depth (all in feet) and divide by 27 for cubic yards. A 50 ft trench, 12 inches wide and 18 inches deep, takes about 2.8 cubic yards, roughly 3.75 tons of #57 stone.
- What size gravel is best for a French drain?
- Clean, washed stone around 3/4 to 1 1/2 inches, such as #57. It has no fines to clog the pipe or the fabric. Avoid crusher run and pea gravel with lots of sand.
- How deep and wide should a French drain be?
- Many yard drains are about 6 to 12 inches wide and 18 to 24 inches deep. Drains near foundations can be deeper. Slope the bottom about 1 inch for every 8 feet toward the outlet.
- Should I subtract the pipe from the gravel volume?
- You can. A 4 inch pipe takes up about 0.09 cubic feet per foot of trench, around 6 percent of a 12 × 18 inch trench. Leaving the 10 percent waste allowance in covers it.