Rainwater Harvesting

Estimate roof runoff captured from a rainfall event.

Your numbers

Potential collected water

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Estimates collected water from one rainfall event using your collection efficiency and remaining tank space. Does not forecast rainfall or assess water quality and drinking safety.

Formula

Use V = Are. A is roof catchment area in m², r is rainfall in mm, and e is collection efficiency as a fraction. V is potential collected litres; actual storage is capped by entered spare tank capacity.

Estimate roof runoff captured from a rainfall event.

Worked example

Roof catchment area (m²): 100
Rainfall depth (mm): 20
Collection efficiency (%): 80
Available tank capacity (L): 1000

Potential collected water: 1,600 L
Stored with entered spare capacity: 1,000 L
Overflow: 600 L

Scope and limitations

Estimates collected water from one rainfall event using your collection efficiency and remaining tank space. Does not forecast rainfall or assess water quality and drinking safety.

Inputs are checked for missing values and allowed ranges. A result outside the finite numeric range is not displayed. Rounding affects display, while the calculation uses the model’s numeric precision.

Check your units and assumptions before relying on a result. Changing a label does not convert a currency or physical unit.

Formula version: 109454605e61 · Example checked: 2026-10-30

Methodology