French drain calculator

Estimate outputs: Drainage gravel, Linear length, Filter fabric. Adjust Trench length, Trench width, Trench depth to match your project.

Inputs and units

Use the units shown beside each field. Replace the sample values with your measurements and the figures on the selected product packaging.

Parameter	Example input
Trench length	20 m
Trench width	0.4 m
Trench depth	0.6 m
Pipe diameter	110 mm
Waste allowance	10 %

Formula and assumptions

Drainage gravel
max(0, Trench length × Trench width × Trench depth - π × (Pipe diameter / 2000)² × Trench length) × (1 + Waste allowance / 100)

Filter fabric
Trench length × (2 × Trench width + 2 × Trench depth) × (1 + Waste allowance / 100)

Linear length
Trench length × (1 + Waste allowance / 100)

ceil = round up; floor = round down; max = the greater value. Trigonometric angles are converted from degrees to radians.

Allowance is applied before rounding to whole packages. Whole units are rounded up; displayed decimals are rounded only for readability.

Bulk density and compaction vary with moisture and the delivered material. The allowance does not model compaction; obtain the ordering conversion from your supplier.

Worked example

Sample values are examples, not product specifications. Run the calculation again with your own measurements.

Result	Calculation
Drainage gravel	5.0709 m³
Linear length	22 m
Filter fabric	44 m²

Related calculators

Related tools use separate measurements. Check their inputs before transferring quantities.