How Much Gravel for a French Drain? (2026 Guide)
How much gravel for a French drain? Multiply the trench volume by 0.85 for the pipe and settling: a 12×18 in trench, 40 ft long, needs ≈ 51 cu ft ≈ 1.9 yd³.
The gravel math, step by step
We'll size a common setup: a trench 12 in wide × 18 in deep running 40 ft. Drain rock fills the excavated trench around a perforated pipe, so the estimate is a simple volume problem.
1. Find the excavated volume in cubic feet
Convert the cross section to feet: 12 in = 1 ft wide and 18 in = 1.5 ft deep, so each linear foot of trench holds 1 × 1.5 = 1.5 cu ft. For 40 ft: 40 × 1.5 = 60 cu ft of excavated space.
2. Apply the 0.85 fill factor
You don't refill the whole trench with stone. The drain pipe takes up part of the volume and gravel settles once it's compacted and watered in, so order about 85% of the excavated total: 60 × 0.85 = 51 cu ft of gravel.
3. Convert to cubic yards
Bulk gravel is sold by the cubic yard (27 cu ft = 1 yd³), so 51 ÷ 27 ≈ 1.9 yd³. For a 40 ft run, order about 2 yd; the small overage covers settling and any low spots after the first heavy rain.
Working with a different width, depth or length? The french drain gravel calculator takes the trench in inches or feet and handles every unit conversion for you.
Gravel for common 12×18 in trench lengths
Estimates use the 0.85 fill factor on a 12 in wide, 18 in deep trench:
| Trench length | Excavated volume | Gravel at 85% fill | Gravel needed |
|---|---|---|---|
| 40 ft | 60 cu ft | 51 cu ft | ≈ 1.9 yd³ |
| 80 ft | 120 cu ft | 102 cu ft | ≈ 3.8 yd³ |
| 120 ft | 180 cu ft | 153 cu ft | ≈ 5.7 yd³ |
| 150 ft | 225 cu ft | 191 cu ft | ≈ 7.1 yd³ |
Check the first row against the worked example: 60 × 0.85 = 51 cu ft ≈ 1.9 yd³, so a 40 ft run means ordering about 2 yd. Digging a wider trench? At the same 18 in depth, an 18 in wide trench takes about 50% more gravel than the 12 in wide one (1.5× the cross section), so scale these totals by 1.5.
Bagged gravel or bulk by the yard?
Garden centers sell small bags of drain rock at about 0.5 cu ft each, so the 40 ft trench works out to 51 ÷ 0.5 = 102 bags — a lot of lifting and trips. Bagged stone only makes sense for a short run or a spot repair; for anything near a full 40 ft trench, order bulk gravel by the cubic yard (about 1.9 yd³) and have it dumped close to the trench.
French drain tips that keep water moving
- Buy the pipe and fabric separately — the gravel total covers only the stone bed; perforated drain pipe, couplings and geotextile filter fabric are priced on their own.
- Hold about a 1% slope — roughly 1 in of fall per 8 ft toward the outlet, so water keeps moving instead of pooling in low spots.
- Use 3/4 in washed gravel — clean stone in the 3/4 in range is the usual pick: open enough to drain fast, stable enough to stay put. Skip sand and crusher dust, which wash fines into the pipe and clog it.
Gravel shows up in plenty of other jobs — see how much gravel for a driveway and the base material math in how much material for a 10×10 shed pad.
FAQ
How many yards of gravel do I need for a 40 ft French drain?
About 2 cubic yards. A 12×18 in trench, 40 ft long, excavates 60 cu ft; with the pipe in place you fill it to about 85%, so the gravel works out to 51 cu ft ≈ 1.9 yd³. Order 2 yd and use the spare to top off after settling.
What size gravel should I use for a French drain?
3/4 in washed stone is the standard choice. Clean gravel in the 3/4 in range — roughly 1/2 to 1 in also works — drains fast and stays put, while sand and crusher dust trap water and wash fines into the pipe.
Do I need filter fabric around a French drain?
Yes. Line the trench with geotextile filter fabric and wrap it over the gravel after backfilling, so water passes through but silt and soil stay out of the stone and pipe. Skip it and a clayey soil will gradually clog the drain.
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