GARDEN & OUTDOOR

Rainwater Harvesting Calculator

Estimate how much rainwater you can collect from your roof. Enter your roof area, total rainfall and collection efficiency to calculate your potential water supply.

Roof area

Enter your roof dimensions or an area you have already measured.

Don't know your roof area? Use our Room Area Calculator ↗ to calculate the area.

Rainfall information

Enter the total accumulated rainfall for the period you want to estimate.

For multiple days, add the daily rainfall amounts together. For example, 10 mm + 15 mm + 20 mm = 45 mm total rainfall. Use accumulated rainfall, not the probability of rain.

Need rainfall information?

Check a local weather service for observed or forecast rainfall in your area.

Collection efficiency

Not all rainwater falling on a roof can be collected. Some is lost through evaporation, splashing and runoff.

An 80% efficiency is used as a starting estimate. You can adjust it based on your roof and collection system.

How is rainwater calculated?

The calculator estimates the amount of rainwater that can be collected from your roof using its area, rainfall and collection efficiency.

View the calculation formula

Potential rainwater (litres):

Roof area (m²) × Rainfall (mm)

Estimated collectible water (litres):

Roof area (m²) × Rainfall (mm) × Collection efficiency

One millimetre of rainfall over one square metre of roof produces one litre of water before collection losses.

Disclaimer: This calculator provides an estimate. Actual collection may vary depending on roof material, slope, drainage, first-flush arrangements, rainfall distribution and collection system efficiency.

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