French drain and soakaway calculator

How it works

Enter the trench dimensions and product-specific density to estimate gravel, pipe and membrane. Choose a specified storage target, or calculate design-storm storage from drained area, rainfall depth, storm duration, a tested infiltration rate and effective soakaway side area.

The storm method follows the inflow-minus-outflow equation in England’s Approved Document H3. Use rainfall and infiltration inputs from the applicable drainage design, compare all required storm durations and use the largest storage result. Recheck the effective side area against the final crate arrangement.

Formula

• Trench materials

  Trench volume = length × width × depth Aggregate mass = trench volume × supplier density Membrane area = length × (width + 2 × depth) × (1 + overlap %)

  The membrane estimate covers the base and two sides. It does not add end panels, pipe penetrations or installation-specific cuts.

• Crates

  Crate count = ceil specified storage target ÷ usable litres per crate

  Enter a target storage volume from the drainage designer and the manufacturer’s usable capacity.

• Design-storm storage

  Inflow (L) = drained area (m²) × rainfall (mm) Outflow (L) = effective side area (m²) × tested infiltration (m/s) × duration (s) × 1,000 Storage = max(0, inflow − outflow)

  Use the drainage design’s effective side area to half-depth, excluding the base. This checks an entered storm; repeat for all required events and confirm emptying, groundwater and location requirements.

Worked example

1. 1

   For a 10 m trench that is 0.3 m wide and 0.4 m deep, geometric volume is 10 × 0.3 × 0.4 = 1.2 m³. At a supplier-entered density of 1.6 t/m³, the arithmetic gives 1.92 tonnes of aggregate. A 5,000 L target with crates rated at 348 usable litres each gives ceil(5,000 ÷ 348) = 15 crates. These are example inputs, not design recommendations.

Assumptions and limits

This is a storm-volume check and material take-off, not an automatic infiltration-site approval or complete drainage design. Rainfall, effective area, tested infiltration, climate allowance, critical duration, emptying time, groundwater, contamination and site setbacks must be established independently.

The source document says infiltration devices should not be built within 5 m of a building or road, in unstable land, where the water table reaches the device, or where contamination could pollute groundwater. Check current rules and the actual site, including underground services, before excavation.

Frequently asked questions