By Zencalcy Editorial TeamLast reviewed About us
A French drain is a trench filled with gravel around a perforated pipe. The takeoff is mostly gravel: the trench volume minus the pipe, converted to tons. Fabric is measured as the trench length times the width plus both side walls, so it can line the trench before gravel goes in.
Gravel, pipe and fabric for a French drain trench.
Gravel = (W × D − π × (d/2)²) × L Fabric = L × (W + 2D)
The pipe displaces its own volume and is usually laid in the lower third of the trench, so the displacement is small. Gravel density is taken as 1,680 kg/m³ and 10% is added for settling. Fabric should be wide enough to drape over both side walls and overlap at the top.
| Symbol | Meaning |
|---|---|
| W | Trench width. |
| D | Trench depth. |
| d | Pipe outside diameter. |
| L | Trench length. |
The trench holds 100 cu ft; the pipe displaces 4.4 cu ft, so with settling the gravel is about 105 cu ft, or 3.9 cu yd and 5.5 US tons.
Continue with the Gravel Calculator, Sand Calculator, Rock Calculator, or the River Rock Calculator — or browse more materials & quantities and all construction calculators.
Fill to within 2–4 inches of the surface, then cap with soil and sod. That is roughly 0.17 cu yd per linear foot for a 12 × 24 inch trench.
Results use standard construction estimating arithmetic: volumes from measured area and depth, unit weights from published material densities, and code-referenced spans or clearances where the page names a code. Quantities are rounded up to whole saleable units and include the stated waste allowance. These tools are planning aids, not structural design, and do not replace a licensed engineer, a local code check, or a contractor's site measurement.
Reference material used to build and check this calculator:
Last reviewed: September 2026. See how we build and check every tool on the about page.
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Content updated: September 2026