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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.
This calculator estimates the drainage gravel needed to fill a French drain trench, accounting for the volume occupied by the drain pipe.
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 × 1.45 t/m³ (drainage rock)
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Gravel Cost Calculator
Estimate total gravel cost from volume with waste factor. Price by cubic meter or tonne with an itemized cost breakdown table.
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Total length of the French drain trench
Trench width — typically 300–600mm
Trench depth from surface — typically 0.3–1.2m
Add 10–15% for spillage and uneven trench walls
With 10% waste
Drainage rock at 1.45 t/m³
L × W × D
π × (OD/2)² × L
Trench minus pipe displacement