How this instrument works
A French drain is a trench filled with gravel or rock, typically wrapped around a perforated pipe, that redirects surface water or groundwater away from an area — commonly used to keep water away from a foundation, replace an open drainage ditch, or relieve groundwater pressure behind a retaining wall. Water enters through the gaps between the gravel and the pipe's perforations, then travels along the pipe to wherever it's meant to discharge.
This instrument treats the trench as a simple rectangular volume: length times width times depth, converted to cubic yards, then multiplied by a gravel density to get the tons of stone needed to fill it. The perforated pipe length is set equal to the trench's straight-run length — the standard simplifying assumption for a drain running in one direction without branches, since the pipe travels the same distance the trench does.
Gravel density is a typical, adjustable figure here, not a fixed constant — crushed stone and gravel under 2in commonly run around 1.4 tons per cubic yard, but the real range spans roughly 1.35 to 1.7 tons per cubic yard depending on the specific stone type and how wet it is. This calculator uses the same density default as this site's other gravel and crushed-stone calculators; adjust it to match whatever stone you're actually sourcing.
- Enter Trench length (ft) — the straight-run distance the drain covers.
- Enter Trench width (in) and Trench depth (in) — the trench's cross-section dimensions.
- Enter Gravel density (tons/yd³) — defaults to 1.4, typical for crushed stone/gravel under 2in; real range is roughly 1.35-1.7 by stone type.
- Read Gravel needed (tons) and Perforated pipe needed (ft) — the fill volume converted to weight, and the pipe length matching the trench's straight run.
Worked example — a 50 ft trench, 12 in wide, 18 in deep
A 50 ft French drain trench is dug 12in wide and 18in deep, filled with typical crushed stone at 1.4 tons per cubic yard. Enter 50 into Trench length (ft), 12 into Trench width (in), 18 into Trench depth (in), and 1.4 into Gravel density (tons/yd³). Converting to feet, width is 1ft and depth is 1.5ft, so volume is 50 × 1 × 1.5 = 75 cu ft, which is 75 ÷ 27 = 2.7778 cubic yards.
Gravel needed (tons) comes to 2.7778 × 1.4 = 3.8889 tons. Perforated pipe needed (ft) simply matches the trench length: 50ft of pipe for this straight run.
Questions
Why does the pipe length just equal the trench length?
Because this calculator assumes the common case of a single straight-run drain — the perforated pipe travels the full length of the trench it sits in, with no branches or turns to add extra pipe. If your drain design includes branches, a curved run with extra fittings, or a discharge extension beyond the gravel-filled trench itself, add that extra pipe length on top of this calculator's figure.
What gravel density should I use?
1.4 tons per cubic yard is a typical default for crushed stone or gravel under 2in, matching this site's other crushed-stone and gravel calculators, but the real range commonly cited runs roughly 1.35 to 1.7 tons per cubic yard depending on the specific stone type, size, and moisture content. If you know the density of the specific product you're buying, use that instead of the default for a more accurate tonnage.
Does this account for the volume the pipe itself displaces?
Not separately — this calculator treats the trench as fully filled with gravel at the entered dimensions, without subtracting the small volume the pipe occupies. For most residential French drain projects the pipe's displaced volume is a small fraction of the total trench volume and safe to ignore, but for very narrow trenches or large-diameter pipe, subtracting the pipe's own volume gives a slightly more precise gravel figure.
What size pipe and trench does a typical French drain use?
Sizing varies by how much water the drain needs to handle and local site conditions, but residential French drains commonly use a trench somewhere in the 12-18in wide, 18-24in deep range with 4in perforated pipe, wrapped in landscape fabric to keep soil from clogging the system over time. Larger drainage needs or longer runs may call for a wider trench or larger pipe — check with a local drainage contractor for sizing specific to your site's water volume.
Should the trench be sloped?
Yes — a French drain typically needs a slight downward slope along its length so water actually flows toward the discharge point rather than pooling in the trench, commonly cited around a 1-in-100 to 1-in-200 grade (roughly 1-2in of drop for every 10ft of trench length). This calculator only computes fill volume and pipe length, not slope, so plan and check your trench's grade separately during excavation.