French Drain Calculator
Estimate project materials from measurements and product coverage. Verify requirements before ordering.
Calculate materials
Planning estimate only; check local codes and product specifications.
Measure the gravel envelope around the drain
- Measure trench length in feet. Enter gravel-envelope width and depth in inches.
- Use the depth occupied by gravel, not automatically the full excavation depth.
- Choose an allowance and calculate cubic yards; keep pipe, fabric and restoration in separate quantities.
French Drain Calculator: formula and assumptions
Gravel (yd³) = trench length × (trench width in inches ÷ 12) × (gravel depth in inches ÷ 12) × (1 + waste ÷ 100) ÷ 27.
Worked calculation
An 80 ft trench, 12 in wide and 12 in deep, with 10% waste requires 3.26 yd³.
What this estimate includes
This is gross rectangular gravel-envelope volume. It does not deduct pipe displacement or design drainage slope, outlet, fabric or pipe capacity.
Planning details that change the estimate
The result is a gross rectangular envelope and does not subtract pipe displacement. It also does not design an outlet, determine slope or assess whether a French drain solves the drainage problem. Irregular trenches should be broken into sections. Locate underground utilities before excavation and confirm a workable discharge route before purchasing materials.
French Drain Calculator FAQs
Is pipe displacement deducted?
No. The displayed volume is the full rectangular gravel envelope plus allowance.
Is gravel depth the same as trench depth?
Only when gravel occupies the full depth. Soil cover or other layers should be excluded from gravel depth.
Does this size the drain pipe?
No. Pipe sizing and discharge performance are separate design questions.
Continue planning this project
Compare bulk material quantities with the Gravel Calculator. Check rectangular volumes with the Cubic Yard Calculator and estimate backfill outside the gravel envelope with the Fill Dirt Calculator.
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