Gravel Calculators
French Drain Gravel Calculator
A French drain is a trench: length × width × depth of the hole you dig, not the lawn above it. Pre-set to washed #57 stone — the drainage grade — at a 12" × 18" trench profile.
A note on this estimate
Pipe displacement (roughly 6% of the trench volume) and the filter-fabric and slope design are not computed here. Fines must stay out of drainage stone — a drain built with crusher run is a drain that clogs.
Last updated: 8 November 2026 · Report an error
How the trench math works
The whole trench gets filled with stone (pipe displaces a little — see note below), so volume is simply length × width × depth of the trench. A 50-foot run, 12 inches wide and 18 inches deep: 50 × 1 × 1.5 = 75 ft³ = 2.78 yd³ of washed #57 ≈ 3.75 tons.
The pipe subtracts less than you'd guess: 50 feet of 4" pipe displaces only ~4.4 ft³ (about 6% of that trench) — most people just let it be part of the safety margin.
Trench sizes by pipe diameter
The trench has to do two jobs: hold the pipe with a working envelope of stone around it, and store water long enough for it to move away. As a rule the trench is at least 3 inches of gravel on every side of the pipe — under it as bedding, beside it, and over the top before fabric and soil close the trench.
| Pipe diameter | Trench width | Trench depth | Gravel per 10 ft of run |
|---|---|---|---|
| 3 in | 12 in | 18 in | ≈ 0.75 t of #57 |
| 4 in | 12–18 in | 18–24 in | ≈ 0.75–1.5 t of #57 |
| 6 in | 18–24 in | 24 in | ≈ 1.5–2.0 t of #57 |
Worked example: a 25 ft drain in a trench 12 in wide and 18 in deep is 25 × 1 × 1.5 = 37.5 ft³ = 1.39 yd³ of washed #57 stone, which is about 1.87 tons. The pipe itself displaces only about 6% of that, well inside the normal ordering margin, so the calculator's trench-volume figure stands as the order quantity. Slope is the other half of the design: the run needs a steady fall of about 1% (⅛ in per foot) or the best-sized trench in the world just stores water in place.
The mistakes that kill French drains: building with crusher run or any stone carrying fines — the voids seal and the drain becomes a buried gravel strip; skipping the filter fabric, so soil migrates in and does the same job more slowly; laying the pipe directly on the trench floor with no bedding stone under it; and running the line level "because the ground looks flat". Every one of those is a materials-or-method error, not a quantity error — no calculator can save a drain built with the wrong stone.
Related tools & guides
Gravel Calculator
Any area, any material — the general tool.
Retaining Wall Gravel Calculator
The other drainage job: backfill behind a wall.
Paver Base Calculator
The same washed-stone logic under flatwork.
Patio Gravel Calculator
Drain the area under a gravel patio too.
Gravel Types and Sizes Guide
Why washed stone drains and dense-graded stone doesn't.
Cubic Yards to Tons Gravel Calculator
Plain tonnage with the density table.
French drain FAQ
What size gravel for a French drain?
Washed #57 stone (about ¾ inch) is the standard choice — large enough for water to move through quickly, small enough to surround the perforated pipe. Never use crusher run or anything with fines: fines wash into and clog the pipe.
How wide and deep should a French drain trench be?
A typical residential trench is 12–18 inches wide and 18–24 inches deep, holding a 4-inch perforated pipe on a bed of stone with stone covering it. Enter your actual trench dimensions — volume comes straight from the trench, not the ground above it.
How much gravel do I need for a 50 foot French drain?
A 50-foot trench, 12 inches wide and 18 inches deep, is 75 cubic feet — about 2.8 cubic yards or roughly 3.75 tons of washed #57 stone. The calculator gives exact figures for your dimensions.
Does the calculator subtract the space the pipe takes up?
No — stone is figured on the full trench volume. A 4-inch pipe displaces only about 6% of a typical trench, and that small surplus works as ordering margin rather than a shortfall, so the calculator leaves it in.
What goes under and over the pipe?
A few inches of washed stone under the pipe as a bed, then stone around and over it until the pipe is well covered, all wrapped in filter fabric so soil cannot migrate in and clog the stone. The trench is then capped with soil or turf.