The formula, your substituted values, and the evaluated result are shown below.
This is a planning estimate. Confirm dimensions, allowances, material specifications, applicable codes, and supplier requirements before purchasing or construction.
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How the French Drain Calculator works
Estimate french drain volume.
Before using the result
This is a planning estimate. Confirm dimensions, allowances, material specifications, applicable codes, and supplier requirements before purchasing or construction.
A French drain is a gravel-filled trench, usually with a slotted pipe, that collects groundwater and carries it away. The calculator gives the trench volume, which is the basis for your gravel order:
Enter every dimension in metres, so a 300 mm wide trench is 0.3.
Worked example: a 15 m drain along a retaining wall
Trench: 15 m long × 0.3 m wide × 0.6 m deep = 2.7 m³
Less the 100 mm pipe: π × 0.05² × 15 ≈ 0.12 m³
Gravel volume: about 2.58 m³
At roughly 1,600 kg/m³ for 20 mm drainage aggregate: 2.58 × 1,600 ≈ 4.1 tonnes
How to build it well
Fall: give the pipe a steady fall to a legal discharge point, typically at least 1% (10 mm per metre). More is better on clay.
Fabric: wrap the gravel in geotextile, or use a socked pipe, so fine soil doesn't clog it.
Gravel: use clean, single-size aggregate (often 10–20 mm), not road base.
Outlet: discharge to an approved stormwater point or absorption area, never onto a neighbour's land. Stormwater connections may need a licensed plumber.
Inspection: fit a cleanout or inspection opening at the high end so the pipe can be flushed.
Typical sizes
Situation
Typical trench
Soggy lawn or garden
150–300 mm wide, 300–450 mm deep
Behind a retaining wall
300 mm wide for the full wall height
Along a house footing
as advised by your engineer; don't undermine footings
Common mistakes
A flat or back-falling pipe that holds water instead of draining.
Leaving out the geotextile, so the drain silts up within a few years.
Digging next to footings without advice. That can weaken the house foundations.
How deep should a French drain be?
Commonly 300–600 mm, deep enough to intercept the water, with the pipe near the bottom and a consistent fall.
What gravel is best for a French drain?
Clean, washed, single-size aggregate about 10–20 mm. Avoid fines, which block water flow.
Does a French drain need a pipe?
Short drains can work with gravel alone, but a slotted pipe carries far more water and is standard for longer runs.
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