French Drain Calculator
Calculate the drainage gravel needed for a French drain trench. Accounts for pipe outer diameter displacement using the same formula as engineering references: π × (OD/2)² × length.
Part of the Landscaping & Site Work calculators — see all tools in this category.
How it works and assumptions
This calculator owns the aggregate takeoff for a specified French-drain trench: it estimates drainage gravel after subtracting the volume occupied by one drain pipe. It is not a drainage design, soil/infiltration assessment, outlet approval, or material specification.
Enter specified inputs
- Enter trench length: Total length of the French drain run in metres.
- Enter trench width: Use the width established by the project drainage detail.
- Enter trench depth: Use the specified depth from the site drainage plan.
- Select pipe size: 100mm, 150mm, or 200mm outer diameter. Or enter a custom diameter.
- Set waste factor: Use an entered ordering allowance that matches the supplier, placement method, and project requirements.
Formula
Trench volume: Length × Width × Depth
Pipe displacement: π × (OD/2)² × Length
Net gravel: Trench Volume − Pipe Displacement
Order quantity: Net Gravel × (1 + Waste %)
Weight: Order quantity × effective drainage-rock density shown in results
Next decisions belong to other pages
Confirm material and placement assumptions with the backfill material guide and the project drainage detail. For an aggregate budget, take this quantity to the gravel cost calculator and enter current local delivered prices rather than relying on a generic cost claim.
If the specified trench requires excavation or worker entry, use the excavation volume calculator for the cut quantity and the OSHA trench safety guide for the applicable excavation requirements. Neither quantity result selects protective measures or confirms that trench conditions are safe for entry.