Rainwater Harvesting Calculator
Rainwater collection uses the simple identity that 1 millimetre of rain falling on 1 square metre of horizontal surface equals exactly 1 litre.
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How to use this calculator
- Enter your roof catchment area in m² — use the horizontal (plan/footprint) area, not the sloped surface area, because roof pitch does not increase how much rain lands on it.
- Enter the rainfall depth in mm — use annual rainfall for a yearly yield, monthly for a monthly estimate, or a single storm depth for one event.
- Optionally set a runoff coefficient between 0 and 1 (default 0.85). Read off the harvestable volume in litres, kilolitres and US gallons plus the litres captured per mm of rain.
How it works
Rainwater collection uses the simple identity that 1 millimetre of rain falling on 1 square metre of horizontal surface equals exactly 1 litre. Multiplying catchment area by rainfall depth therefore gives the gross rain that lands on the roof, and multiplying by a runoff coefficient reduces it to what actually reaches the tank: Volume (L) = roof area (m²) × rainfall (mm) × runoff coefficient.
The runoff coefficient captures real-world losses — first-flush diversion, evaporation on hot dry roofs, splash-out, gutter overflow in heavy downpours and absorption by roofing materials. Smooth metal roofs run high (0.85–0.95); tiles, concrete and rougher surfaces run lower (0.70–0.85). Always use the plan (footprint) area rather than the sloped area: a steeper pitch presents more surface but catches the same horizontal rainfall, so the two effects cancel out.
Worked example
150 m² roof, 800 mm annual rain. A house with 150 m² of roof plan area, 800 mm of annual rainfall and a runoff coefficient of 0.85 has a harvestable volume of 150 × 800 × 0.85 = 102,000 L, or 102.00 kL (m³) — about 26,946 US gallons. Each 1 mm of rain yields 127.5 L, so a single 20 mm downpour delivers roughly 2,550 L into the tank.
Common mistakes
- Using the sloped roof surface area instead of the horizontal plan (footprint) area. Rain falls vertically, so only the footprint matters — using the sloped area overstates your yield.
- Assuming 100% capture. Without a runoff coefficient you overstate real yield by 10–30%; first-flush, overflow and evaporation always take a share.
- Mixing units — entering rainfall in cm or inches instead of mm, or roof area in ft² instead of m². The 1 mm × 1 m² = 1 L identity only holds in metric.
Frequently asked questions
Should I use the flat roof area or the sloped surface area?
Use the flat horizontal plan area (the building footprint under the roof). Rain falls vertically, so a steep roof catches the same volume as a flat one of the same footprint — a sloped area figure will overstate your yield.
What runoff coefficient should I use?
For a clean metal (Colorbond/steel) roof, 0.85–0.95 is typical. Tiled, concrete or rougher roofs lose more to absorption and splash, so 0.70–0.85 is realistic. The calculator defaults to 0.85, a sensible middle value if you are unsure.
Does 1 mm of rain really equal 1 litre per square metre?
Yes — it is exact by definition. 1 mm depth over 1 m² is 0.001 m × 1 m² = 0.001 m³, and 0.001 m³ is exactly 1 litre. That is why the metric formula is so clean.
How does this relate to my tank size?
This tool estimates how much rain your roof can capture over a period; it does not size the tank. Your tank only needs to hold what you collect between top-ups and draw-downs, so pair this with a tank volume calculator and your usage pattern to choose a capacity.
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Tip: Enter any known values to calculate the remaining results.
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