Rainwater Harvesting Calculator

How it works

1. 1Enter your roof catchment area in square metres and your local annual rainfall in millimetres.
2. 2Set the runoff coefficient (0.80–0.90 for hard roofs) to account for splash, evaporation, and first-flush losses.
3. 3The calculator multiplies area × rainfall × coefficient — because 1 mm on 1 m² equals 1 litre — and displays your annual harvest potential.

Use cases

• Sizing a rainwater storage tank for a home or office building.
• Comparing catchment potential across different roof surfaces or materials.
• Estimating water self-sufficiency before installing a harvesting system.

Frequently asked questions

How is the harvest potential calculated?

Harvest (L) = catchment area (m²) × annual rainfall (mm) × runoff coefficient. For example, 100 m² × 600 mm × 0.85 = 51,000 L/year. The formula works because 1 mm of rain falling on 1 m² yields exactly 1 litre.

What runoff coefficient should I use?

Metal or tile roofs typically achieve 0.80–0.90. Concrete surfaces range from 0.70–0.85. Gravel or green roofs can be as low as 0.30–0.50. The default of 0.85 is a good starting point for a hard roof with gutters.

How do I size my storage tank?

Divide the annual harvest by 12 to get the average monthly yield, then size the tank to cover the longest expected dry season. For 51,000 L/year with a two-month dry season, aim for at least 8,500 L of storage capacity.