French Drain Gravel Calculator – How Much Stone? | CalcLoft
Find out exactly how many cubic yards of crushed stone or gravel your French drain trench needs — enter length, width and depth to get cubic yards, tons and bag count in seconds.
How to calculate French drain gravel
Find the volume of the trench in cubic feet, then take about 85% of it as gravel. The ~15% deduction leaves room for the pipe and for the layer of topsoil that covers the gravel back to grade.
Trench volume (cu ft) = length (ft) × width (in) ÷ 12 × depth (in) ÷ 12
For example, a 40 ft run of a 12 × 18 in trench (1 ft wide and 1.5 ft deep) holds 40 × 1 × 1.5 = 60 cu ft. Gravel is about 60 × 0.85 = 51 cu ft, which is 1.9 cu yd (51 ÷ 27).
Gravel for a 12 in × 18 in trench
| Trench length | Trench volume | Gravel (~85%) | Gravel needed |
|---|---|---|---|
| 40 ft | 60 cu ft | 51 cu ft | 1.9 cu yd |
| 80 ft | 120 cu ft | 102 cu ft | 3.8 cu yd |
| 120 ft | 180 cu ft | 153 cu ft | 5.7 cu yd |
| 150 ft | 225 cu ft | 191 cu ft | 7.1 cu yd |
Buying gravel by the yard, ton or bag
Washed drain rock is usually sold in bulk by the cubic yard or by the ton — it weighs about 1.35 tons per cubic yard, so 1.9 cu yd is roughly 2.5 tons. For small jobs, gravel also comes in 0.5 cu ft bags; 51 cu ft ÷ 0.5 = 102 bags for that 40 ft run. Always buy a little extra, because gravel settles once it is tamped into the trench.
An 18 in wide trench holds about 50% more gravel than a 12 in trench at the same depth, so run both widths through the calculator before you order.
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French drain gravel calculator FAQ
Should my French drain trench be 12 or 18 inches wide?
A 12 in wide trench is enough for most 4 in drains — it leaves a solid gravel bed under and around the pipe. Choose 18 in for a 6 in pipe, heavy clay soil, or a long run that carries a lot of water. Note that an 18 in trench needs about 50% more gravel than a 12 in trench at the same depth.
How much gravel will a French drain need?
A 40 ft run of a 12 × 18 in trench takes about 1.9 cu yd of gravel — roughly 2.5 tons, or 102 × 0.5 cu ft bags. A 100 ft run takes about 4.7 cu yd. Enter your trench dimensions above for an exact estimate.
Do I need to wrap the gravel in landscape fabric?
Yes. Line the trench with permeable landscape or filter fabric (or slide a filter sock over the pipe) so silt and clay cannot wash in and clog the gravel. Fabric is sold separately and is not included in the gravel totals above.