Rainwater Planning ExplainedCatchment, storage & reuse
Home / Tools / Rainfall Capture Calculator
Browser calculator

Rainfall Capture Calculator

Estimate how much rainwater a roof can collect before tank overflow and other system-specific losses.

Roof capture calculator

Area draining to the chosen collection point.

Event, monthly or annual depth using the selected units.

Use a conservative factor for wetting, splash and conveyance losses.

Formula

Metric litres = roof area (m²) × rainfall (mm) × runoff factor

US gallons = roof area (ft²) × rainfall (in) × 0.623 × runoff factor

Use conservative inputs

Collection loss varies with roof wetting, wind, splash, debris, gutter performance, first flush and the amount of free storage available. For major projects, compare monthly or daily rainfall with demand rather than relying on one annual number.

Planning estimate only: Verify local rainfall, product ratings, structural support, overflow, plumbing, water-quality and permit requirements.