French drain calculator
Enter the trench — length, width and depth — and get the washed gravel to fill it, minus the space the 4-inch pipe takes.
Result
Drain gravel needed
How much gravel does a French drain need?
Formula
ft³ = length × width (in ÷ 12) × depth (in ÷ 12) · yd³ = ft³ ÷ 27 · tons ≈ yd³ × 1.35
Example — classic yard drain: 60 ft of 12-inch-wide trench, 18 in deep: 60 × 1 × 1.5 = 90 ft³ = 3.3 yd³ ≈ 4.5 tons of #57 washed stone. The 4 in perforated pipe takes ~0.35 ft³ per running foot (about 21 ft³ here), so some finish with gravel only at the top — order the full amount and top-grade with soil if you're pipe-less.
The build, in order
- Dig the trench with at least 1% slope (⅛ in per foot) toward the outlet.
- Line with non-woven geotextile — this is what keeps the drain alive for decades instead of silting shut.
- 2–3 in of washed stone in the bottom, then the 4 in perforated pipe (holes down), sleeves optional.
- Fill with washed stone to 2 in below grade; fold fabric over; cap with soil or decorative stone.
Use washed stone, not crusher run. Fines in crusher run clog the fabric and kill flow. #57 washed (or #4 for high-flow) is the standard. Compute pipe bedding under patios with the paver base calculator.
Frequently asked questions
How much gravel for a 50 foot French drain?
A 12 in wide, 18 in deep trench: 75 ft³ = 2.8 yd³ ≈ 3.8 tons of #57 washed stone, plus ~50 ft of 4 in pipe and fabric.
How deep should a French drain be?
18–24 in handles most yard drainage. Along foundations, keep the pipe at or below footing level and slope to daylight or a dry well.
What size gravel for a French drain?
Washed #57 (¾ in, no fines). Bigger (#4) for high-volume drains; pea gravel works but costs more per ft³ and migrates more.
Do I really need landscape fabric in a French drain?
Yes — non-woven geotextile is the difference between a 30-year drain and one that silts up in 3. Wrap pipe and gravel completely, overlap seams 12 in.
French drain vs drain tile?
Same thing under different names — perforated pipe in a gravel trench. "Drain tile" is the farm-era term.