French Drain Calculator
Plan drainage gravel, perforated pipe, landscape fabric and total fall for a French drain.
Your French drain plan
- Gravel to buy
- cu yd, including waste
- Perforated pipe
- ft
- Pipe diameter
- in
- Landscape fabric
- sq ft
- Minimum fabric roll width
- ft
- Continuous fabric length
- ft
- Required total fall
- in
- Total fall in feet
- ft
- Equivalent slope
- %
- Equivalent drop per foot
- in/ft
- Gravel before waste
- cu ft
- Gravel before waste
- cu yd
- Pipe volume subtracted
- cu ft
Bags to buy: (alternative to bulk gravel)
Shopping list
Plan illustrations
How it works
What is a French drain?
A French drain is a gravel-filled trench, usually containing perforated drainage pipe, that collects unwanted water and carries it toward a safe discharge point.
Enter the length and size of the trench and the slope you plan to use. RockUtility estimates the gravel, pipe, landscape fabric and total elevation drop needed for the project.
We multiply length, width and gravel depth, subtract the pipe volume when it is inside the gravel, then add your gravel waste allowance. Fabric covers the bottom, both sides and top, plus your overlap. Total fall comes from length and slope.
Assumptions and useful information
The trench has a uniform rectangular cross-section and follows the entered grade. Full fill uses the entire depth; custom fill uses your gravel depth. The pipe must fit entirely inside or above the gravel. This does not design bedding, cover or hydraulic capacity.
Pipe length equals drain length, without hidden allowance. Fabric assumes one continuous full-depth wrap; buy extra for seams, ends, bends and installation slack. Choose drainage geotextile suited to your soil and installation instructions. Bag counts use your entered bag volume.
1% is a planning example, not a universal requirement. Verify that a suitable discharge point is lower by the calculated fall. Values are rounded for readability.