Use our rainwater harvesting calculator to estimate how much rainwater your roof can collect from rainfall. Enter your roof area and local rainfall to calculate your potential rainwater catchment yield, helping you choose the right water tank size for your home, garden, lifestyle block, farm, or off-grid property. A simple way to plan rainwater collection, reduce mains water use, and make better use of the water that falls on your roof.
New Zealand Rainwater Harvesting Calculator
Use our rainwater harvesting calculator to estimate how much rainwater your roof can collect, then select a tank size to see how many days of reserve water storage it may provide for your household.
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Click or tap this dropdown to choose the closest region.
Select the closest region to your property. These are approximate average annual rainfall figures.
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Change this to match the roof area that will collect rainwater.
Enter the roof area in square metres. Example: 100 m².
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Change this if your roof, guttering or first-flush setup is different.
Allows for splash, overflow, first flush, gutter loss and general system loss.
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Change this to match how many people live in the home.
Enter how many people live in the home.
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Click or tap this dropdown to change estimated daily water use.
This helps estimate total daily household water demand.
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Choose a tank size to see how many days of reserve water it may provide.
This calculates reserve days based on the selected tank and household water use.
Rainwater collected per year
0 L
Average collection per month
0 L
Average collection per day
0 L
Selected tank
0 L
Reserve water storage
0 days
Estimated household use
0 L/day
Formula: roof area m² × annual rainfall mm × collection efficiency = litres collected per year.
Reserve water storage is calculated as selected tank size ÷ estimated household water use per day.
This New Zealand rainwater collection calculator is an estimate only. Actual results depend on your exact location, roof shape, guttering, downpipes, first-flush system, overflow setup, rainfall pattern and water usage.