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: 7 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 ≈ 3.7 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 50 ft drain in a trench 12 in wide and 18 in deep is 50 × 1 × 1.5 = 75 ft³ = 2.78 yd³ of washed #57 stone, which is 3.75 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
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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
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Gravel Calculator (Tons)
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 3.7 tons of washed #57 stone. The calculator gives exact figures for your dimensions.