French Drain Calculator
Trench Dimensions
Materials & Cost Factors
Includes standard guidelines recommending round drainage gravel (1/2" to 1-1/2" washed stone) wrapped entirely in non-woven geotextile filter fabric to prevent soil clogging.
French Drain Calculator: How Much Gravel, Pipe, and Fabric Do You Need?
A French drain moves water away from your foundation or yard using a gravel-filled trench and perforated pipe — but the gravel volume isn't as simple as trench length times width times depth, since the pipe itself displaces space. Our free French drain calculator accounts for that automatically, along with landscape fabric and labor, to give you a complete materials estimate.
How the French Drain Calculator Works
Gravel backfill volume is calculated as your total trench volume minus the volume displaced by the pipe itself, so you're not over-ordering gravel to fill space the pipe already occupies. Perforated pipe length matches your trench length, with a waste allowance for cuts and fittings. Landscape fabric coverage wraps the trench bottom and both side walls, plus an overlap allowance, since fabric is typically installed to line the entire trench before backfilling. All three materials are then multiplied by your local prices, combined with your entered labor hours, to build a full project cost estimate.
Why We Add a Waste Buffer (and How to Change It)
This calculator applies the adjustable waste buffer to the gravel volume, pipe length, and fabric coverage. Trenches are rarely perfectly uniform in depth and width along their full run, pipe needs connectors and occasional cuts at bends, and fabric needs overlap at every seam — this buffer accounts for all of that.
If your trench is short, straight, and consistently dug, or you'd rather order closer to the exact number, you can adjust the waste buffer percentage directly on this calculator — set it anywhere from 0% to 50% to match your comfort level. Lowering it will reduce the material estimate and total cost shown across all three materials.
French Drain Trench Sizing Guide
| Application | Typical Depth | Typical Width |
|---|---|---|
| Simple yard drainage | 12–18 inches | 6–12 inches |
| Foundation drainage | 18–24 inches | 12 inches |
| Heavy water table / clay soil | 24+ inches | 12–18 inches |
Always slope the trench slightly (a common target is about 1% grade) so water actively flows toward a discharge point rather than sitting in the pipe.
Frequently Asked Questions
Why doesn't gravel volume just equal trench volume?
Because the perforated pipe running through the trench takes up space that would otherwise be filled with gravel. This calculator subtracts the pipe's volume from the total trench volume so you don't over-order gravel.
How deep should a French drain be?
Most residential French drains run 12 to 18 inches deep for general yard drainage, with foundation drains often going 18 to 24 inches or more depending on soil and water table conditions.
What size pipe should I use for a French drain?
4-inch perforated pipe is the most common choice for residential drainage. Larger 6-inch pipe is typically reserved for handling higher water volumes or longer drain runs.
Do I need landscape fabric in a French drain?
Yes, in almost all cases. Fabric lining the trench keeps soil and silt from clogging the gravel and pipe over time, which is one of the most common reasons French drains eventually stop working.
What slope does a French drain need?
A gentle, consistent slope — often targeted around 1% — helps water flow toward the discharge point instead of pooling in the pipe. Flat or reverse-sloped sections are a common cause of drainage failure.
Related Calculators
Planning a full landscaping project? These calculators pair well with a French drain:
Slope / Grade Calculator — check your yard's grade before routing a drain line.
Gravel Calculator — estimate gravel needs for other drainage or path areas.
Topsoil Calculator — plan regrading needed alongside your drainage work.