All categories
37 tools in 6 categoriesSoil & Ground Cover
View all 11 tools →Project Costs
View all 6 tools →Water & Drainage
View all 6 tools →Lawn & Grass
View all 5 tools →Trees & Plants
View all 5 tools →French Drain Calculator
Work out gravel volume, drain pipe length and a landscape fabric estimate for a French drain trench, plus the minimum slope it needs to actually drain by gravity.
Trench Dimensions
1. The gravel-filled trench
Measure the trench itself -- length along the drain's run, width across the dug channel, and depth from grade to the bottom of the trench, not just the pipe's diameter.
Typical residential French drain trenches run about 6–12 in wide and 18–24 in deep, but yours may be dug differently -- enter your actual measurements.
2. Backfill stone
Open-graded and washed clean of fines, so it beds in rather than compacting -- the standard French drain backfill.
A French drain is a trench, not a flat bed, so it needs length, width and depth rather than an area and a depth -- and unlike most drainage calculators, gravel volume is only part of the job here. This calculator also works out drain pipe length, a landscape fabric estimate to keep the gravel from silting shut, and the one thing almost no French drain calculator mentions: the minimum slope the trench needs to actually drain by gravity.
Reviewed by Usama, Civil EngineerHow we source our numbers
How it works
- Measure the trench: length along the drain's run, width across the dug channel, and depth from grade to the trench bottom.
- Read the gravel volume in cubic yards and tons of #57 stone, the open-graded washed stone standard for French drains, plus the drain pipe length and a landscape fabric estimate for wrapping the trench.
- Check the slope note against your own site: a French drain only works if it falls toward its outlet, and this calculator works out the minimum total fall your trench length needs at a safe grade.
Why a French drain is a trench calculation, not an area calculation
Mulch, gravel beds and soil are spread across a flat area at a chosen depth, so area times depth is the right formula. A French drain is a buried channel: gravel fills the full length, width and depth of a dug trench, so the correct formula is length times width times depth, the same shape math as a rectangular excavation. Getting the width wrong by even a few inches compounds across the whole trench length, which is why this calculator asks for all three trench dimensions directly rather than assuming a standard width.
Why #57 stone, and why it needs no compaction factor
French drains use a specific open-graded aggregate, commonly specified as #57 stone, a roughly 3/4 to 1 inch clean, washed, angular stone sized under AASHTO M43. Open-graded stone is never rolled or compacted the way a dense-graded driveway base is -- it is placed and left, so its loose density and its placed density are the same figure. That is the same #57 stone option and the same LA County Dept. of Public Works density figures used by BunnyCorner's Gravel Calculator, so the gravel math on this page will match a driveway or path estimate using the same material elsewhere on this site.
The part almost every French drain calculator skips: slope
A French drain works entirely by gravity -- there is no pump moving water through the pipe, so if the trench does not fall consistently toward its outlet, water sits in the gravel instead of draining away, and a flat or back-pitched trench can fail completely even when it is filled with the correct stone. Washington State University Extension recommends sloping the trench between 0.5% and 1%: a 1 foot drop over a 100 foot run is a 1% grade. This calculator converts that percentage into a concrete number of inches of total fall for your specific trench length, since "1%" means nothing actionable on its own without knowing how long the run is.
Pipe, fabric, and what this calculator does not size
Drain pipe length tracks trench length directly, plus whatever extra run is needed to reach a downspout tie-in or a pop-up emitter at the outlet, which this calculator doesn't estimate since it depends entirely on site layout. Landscape fabric wraps the trench bottom and side walls before the gravel goes in, keeping soil fines from migrating into the stone's void space and silting the drain's capacity shut over the years -- use the site's Landscape Fabric Calculator with the wrap area shown here to work out rolls and overlap. Perforated pipe diameter, whether to run solid pipe past a building foundation, and outlet daylighting are all site-specific decisions this calculator leaves to a design plan or a licensed contractor.
Further reading
Frequently asked questions
How much gravel do I need for a French drain?
Multiply the trench's length, width and depth to get cubic feet, divide by 27 for cubic yards, then convert to tons using #57 stone's density of roughly 2,570 to 2,632.5 lb per cubic yard, the LA County Dept. of Public Works figures also used by this site's Gravel Calculator. Enter your own trench dimensions above for an exact figure rather than a per-foot rule of thumb, since trench width and depth vary a lot by project.
What size gravel is used for a French drain?
#57 stone, a roughly 3/4 to 1 inch clean, washed, open-graded stone sized under AASHTO M43, is the standard specification. It's open-graded rather than dense-graded, meaning it's meant to have void space between the stones for water to move through, not to compact into a solid mass the way a driveway base does.
What is the best gravel for a French drain?
Clean, washed, open-graded #57 stone rather than a dense-graded material like crusher run, since a French drain needs the void space between stones to let water move through it. A dense-graded aggregate compacts and fills its own voids, which defeats the purpose of the drain.
How deep should a French drain be?
Typical residential trenches run about 18 to 24 inches deep, though the right depth depends on what the drain is intercepting -- a foundation perimeter drain often goes deeper than a simple yard drainage trench. This calculator uses whatever depth you enter rather than assuming one, since the correct depth is a site-specific design decision.
How wide should a French drain trench be?
About 6 to 12 inches is typical for a residential trench, wide enough to fit the pipe with gravel on all sides. Enter your actual planned width above, since a narrower or wider trench changes the gravel volume directly and proportionally.
What slope does a French drain need?
A minimum of about 0.5% to 1%, per Washington State University Extension -- a 1 foot drop over a 100 foot run is a 1% grade. A French drain works entirely by gravity, so a flat or back-pitched trench simply will not drain, no matter how much gravel fills it. This calculator converts that percentage into inches of total fall for your specific trench length.
How much fall does a French drain need per foot?
At the WSU Extension-recommended 1% grade, that's about 0.12 inches of fall for every foot of trench, or roughly 1 inch of fall every 8 feet. This calculator works out the total fall your specific trench length needs at both the 0.5% minimum and the more reliable 1% target.
What size pipe do I need for a French drain?
4 inch perforated pipe is the common residential standard, wrapped in a filter sock or landscape fabric to keep gravel and soil out of the perforations. This calculator estimates pipe length, roughly equal to trench length, but pipe diameter is a site-specific decision this tool doesn't size.
Do I need landscape fabric for a French drain?
Yes, in almost every installation. Fabric wraps the trench bottom and side walls before the gravel goes in, keeping fine soil particles from migrating into the stone's void space and silting the drain's capacity shut over the years. This calculator estimates the wrap area; use the Landscape Fabric Calculator with that figure to work out rolls and overlap.
How long does drain pipe need to be for a French drain?
Roughly equal to the trench length, plus whatever extra run is needed to reach a downspout tie-in or a pop-up emitter at the outlet. This calculator's pipe length figure covers the trench run itself; add extra for those site-specific connections.
What's the difference between a French drain and a regular gravel bed?
A French drain is a buried trench solving a water-management problem: length times width times depth of gravel around a perforated pipe, sloped to drain by gravity. A gravel bed or path from the Gravel Calculator is a flat, spread surface: area times depth, with no pipe and no slope requirement. Both can use the same #57 stone, but the shape of the calculation and the drainage function are different.