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Rainwater Harvesting Potential Calculator

Work out rainwater harvesting potential instantly with clear inputs, formula shown and shareable results.

About Rainwater Harvesting Potential Calculator

One millimetre of rain on one square metre is one litre, so the harvest is area times rainfall times collection efficiency. A 120 square metre roof under 750 mm of annual rain at 85% efficiency yields 76,500 litres a year - about 210 litres a day, or 219 days of a 350 litre household demand.

Formulas

Harvest volume

litres = area(m2) x rainfall(mm) x efficiency

Key identity

1 mm of rain on 1 m2 = 1 litre

Worked example

With the default values below, Rainwater Harvesting Potential Calculator returns Annual Water Harvest of 76,500. Change any input above and every figure here is recalculated in your browser.

Inputs used

Collection areaminimum 0 m2
120 m2
Annual rainfallminimum 0 mm
750 mm
Collection efficiencyaccepts 0–1
0.85

Results

Annual Water Harvest
76,500 L
Same Volume In Cubic Metres
76.50
Daily Average Yield
209.4 L
Days Of Supply At 350 L A Day
219

Step by step

  1. 1Values used
  2. 2Harvest volume
  3. 3Key identity
  4. 4Annual Water Harvest= 76,500 L
  5. 5Same Volume In Cubic Metres= 76.50
  6. 6Daily Average Yield= 209.4 L
  7. 7Days Of Supply At 350 L A Day= 219

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax or fees or charges into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why is efficiency below one?

First-flush diversion, evaporation, splash loss and gutter overflow during intense storms all reduce capture. 0.8-0.9 is realistic for a hard roof.

How big should the tank be?

Usually sized on the longest dry spell rather than annual yield - typically 20-40 days of demand for a garden supply.

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