French Drain Calculator

Estimate runoff flow, gravel volume, filter fabric, and perforated pipe for a French drain project.

French drain material and flow estimate
Enter the contributing area, design rainfall, runoff factor, and trench dimensions.

About French drain sizing

A French drain is a gravel-filled trench, usually containing perforated pipe, that intercepts subsurface or surface water and conveys it to a suitable outlet. This calculator combines a simplified runoff estimate with a material takeoff. Peak runoff is based on drainage area, rainfall intensity, and a runoff coefficient. The coefficient represents the fraction of rain expected to become runoff: roofs and pavement approach the upper end, while permeable landscaped soil generally uses a lower value. The conversion reports the resulting flow in US gallons per minute. Gravel volume is calculated from the rectangular trench volume after subtracting the cylindrical volume occupied by the pipe. This is a geometric quantity and does not include compaction, settlement, uneven excavation, bedding beyond the entered trench, or delivery loss. The filter-fabric estimate covers the trench bottom and both side walls along its full length. Projects that wrap fabric over the top, overlap seams, or line end faces need additional fabric. Pipe length equals the entered drain route and should be increased for fittings, cleanouts, connections, and routing adjustments. Hydraulic capacity depends on much more than excavation volume. Pipe diameter, perforation pattern, internal roughness, slope, inlet conditions, aggregate void space, soil permeability, groundwater, and outlet elevation all affect performance. The displayed runoff is a planning demand, not a certification that the selected pipe or trench can carry it. Use local intensity-duration-frequency rainfall data for the required design storm rather than a casual rainfall average. Confirm that discharge is legal and will not create erosion, icing, slope instability, or a nuisance on neighboring property. A reliable installation starts with diagnosing the water source and surveying elevations. Maintain a continuous fall toward a daylight outlet, storm connection, dry well, or other approved destination. Use clean, washed, angular drainage stone and compatible geotextile so fine soil does not quickly clog the voids. Call utility-locating services before excavation and protect foundations and slopes. For foundation drainage, large contributing areas, poor soils, retaining walls, or sites without a dependable gravity outlet, consult a drainage professional or civil engineer before construction.

French drain examples

Gravel quantities subtract the volume occupied by the perforated pipe.

ProjectTrench and pipeEstimated materials
Yard interceptor100 ft, 12 in by 18 in, 4 in pipe5.23 yd3 gravel and 400 sq ft fabric
Wide collector50 ft, 18 in by 24 in, 6 in pipe5.19 yd3 gravel and 275 sq ft fabric
Foundation run80 ft, 16 in by 24 in, 4 in pipe7.65 yd3 gravel and 426.67 sq ft fabric

How to estimate a French drain

  1. Measure the area that contributes water and obtain a suitable design rainfall intensity.
  2. Choose a runoff coefficient that reflects the surface and soil conditions.
  3. Lay out the drain route and enter the proposed trench and pipe dimensions.
  4. Calculate materials, add overlap and delivery allowances, and verify slope and outlet capacity.

French drain calculator FAQ

What runoff coefficient should I use?

Impervious roofs and paving use higher coefficients than lawns or permeable soil. Select a value from local drainage guidance that matches the contributing surface and design method.

Does gravel volume account for the pipe?

Yes, the calculator subtracts the geometric volume of the round pipe from the rectangular trench. It does not add an allowance for compaction, settlement, spillage, or irregular excavation.

How much slope does a French drain need?

A continuous positive slope is essential for a gravity outlet, and one percent is a common practical target. Site constraints and local requirements can change the appropriate minimum.

Why is filter fabric needed?

Compatible geotextile separates surrounding soil from clean drainage stone and slows migration of fines. Poor fabric selection or incomplete wrapping can contribute to clogging and reduced performance.

Can the estimated runoff size the pipe?

The flow result is a preliminary demand estimate only. Pipe capacity must be checked using diameter, slope, roughness, inlet behavior, aggregate voids, and the intended design storm.