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Rainwater Harvesting Yield Calculator

Enter your roof area, annual rainfall and collection efficiency to see how much rainwater you could harvest.

How it is calculated

One millimetre of rain on one square metre equals one litre, so annual yield (L) = roof area (m²) × annual rainfall (mm) × efficiency / 100. The tool then divides by 12 for the monthly average and by 365 for the daily average. Use the plan (horizontal) area of the roof, the footprint seen from above, not the sloped surface. Efficiency covers losses such as evaporation, splashing, the first-flush diverter and overflow, and must be between 0 and 100 %. The averages spread the water evenly through the year, but real rain is uneven, so the tank has to be sized for dry spells and the figures are not a guarantee for any single month.

Worked example

A roof with a plan area of 80 m², annual rainfall of 650 mm and 85 % collection efficiency. Water falling on the roof = 80 × 650 = 52,000 L. After losses: 52,000 × 0.85 = 44,200 L a year. Monthly average = 44,200 / 12 = 3,683 L. Daily average = 44,200 / 365 = 121 L. A garden using 200 L a day in summer would run past this average, so storage matters.

Try this example →

Common mistakes

  • Measuring the sloped roof surface instead of the footprint: rain falls on the horizontal projection, so the sloped area overstates the yield.
  • Using the area of the whole building or including a garage or porch whose water does not drain to the tank.
  • Entering rainfall in centimetres or inches: the field is millimetres, and 65 cm should be entered as 650.
  • Entering efficiency as 0.85: it is read as 0.85 %, which gives a tiny yield.
  • Treating the daily average as available every day: it hides dry months, so a tank sized to the average will run empty in a dry spell.

About this calculator

The amount of rainwater a roof can collect follows a simple physical relationship: 1 millimetre of rain falling on 1 square metre of roof produces exactly 1 litre of water, so annual yield (litres) = roof area (m²) × annual rainfall (mm) × collection efficiency. Collection efficiency is always less than 100% because some water evaporates, splashes off, or is lost to the “first flush” diverter that clears debris and bird droppings from the roof before water reaches the storage tank — typical real-world efficiencies run 75–90% depending on roof material and system design.

Homeowners, gardeners and sustainability-focused builders use this figure to size a rainwater harvesting system correctly: an undersized tank wastes potential collection during heavy rain, while an oversized one costs more than the realistic yield justifies. It’s also the number used to estimate how much mains water a harvesting system can actually displace over a year for irrigation, toilet flushing or laundry.

How many litres of rainwater you could realistically harvest each year is worked out from your roof area, annual rainfall and collection efficiency.

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