What this calculator does
An infiltration trench (French drain) manages a design storm by storing it in aggregate void space and releasing it into the soil. This calculator sizes the trench from the volume of runoff you must store and tells you how long that volume takes to drain away.
How to use it
- Enter the catchment area and the design rainfall for the storm you must handle.
- Set the runoff coefficient for the surface draining to the trench.
- Set the void ratio of the aggregate bed — 30 to 40 % is typical for washed stone.
- Enter the final infiltration rate from a test, and check the drain time against the interval between storms.
Governing formula
V = A · P · C ; V_trench = V / n ; t = V / (k · A_base) Where:
-
A= Catchment area [m²] -
P= Design rainfall depth [m] -
n= Void ratio of the aggregate [–] -
k= Final infiltration rate [m/h]
Derived Equations:
L = V_trench / (w · d) Worked example — 400 m² of paving with a 25 mm storm
The figures below were produced by running this page's own engine with the example inputs (Catchment area (m²), Design rainfall depth (mm), Runoff coefficient (–), Trench depth (m), Trench width (m), Aggregate void ratio (%), Final infiltration rate (mm/h)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Required trench length | 38.1 m |
|---|---|
| Runoff to store | 8 m³ |
| Drain time | 14000 h |
| Assessment | Store 8.0 m³ (drainage area 400 m² off 25 mm rain) → trench ≈ 38 m long at 0.6 × 1 m (35% voids). Drains in ≈ 14000 h at 25 mm/h. |
- Runoff to store is 400 × 0.025 × 0.8 = 8 m³.
- At 35 % voids the trench must hold 22.9 m³ of aggregate bed.
- A 0.6 × 1.0 m trench therefore needs about 38 m, and drains the volume in roughly 22 hours.
Assumptions & limitations
Engineering calculation. Storage-based sizing of a soakage trench with a Darcy drawdown estimate. No lateral spreading, no groundwater level and no native soil storage.
- Assumes the trench drains from the bottom only; a French drain distributed along the length drains faster.
- No allowance for groundwater mounding, which can seriously reduce infiltration after repeated storms.
- Infiltration rates reduce over time through clogging, so size with a factor of safety.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- Local stormwater infiltration practice notes.
- BS EN 752-4 — underground drainage systems.