Thinking about harvesting rainwater at home? This free calculator estimates how many litres your roof can collect each year and suggests a suitable tank size for your household. Just enter your roof area, annual rainfall and daily water use to see your results instantly.
Rainwater Tank Calculator
How much rain can your roof harvest? Enter your details below — results in litres.
Multiply your roof area by annual rainfall and the runoff coefficient to estimate yearly harvest. We then take three months of average supply as a dry reserve, rounded up to the nearest 1,000 litres, and divide that tank size by your daily use to show coverage. Figures are estimates only; check your council’s rebates, regulations and plumbing rules before buying.
How to use the calculator
- Enter your roof catchment area in square metres — use the horizontal footprint of your roof, not the sloped surface.
- Add your average annual rainfall in millimetres and set the runoff coefficient (0.9 suits most metal roofs; use 0.8 for tiles).
- Press Calculate to see your estimated annual harvest, suggested tank size and how many days of supply it provides.
Frequently asked questions
What is a runoff coefficient?
It is the share of rainfall that actually reaches your tank after losses from evaporation, splashing and overflowing gutters. Most metal roofs rate around 0.9, while tiled roofs are closer to 0.8.
What size rainwater tank do I need?
It depends on your roof size, local rainfall and how much water you use. This calculator suggests a tank that holds about three months of your average harvest, rounded up to the nearest 1,000 litres.
Can I drink rainwater from my tank?
Tank water is great for gardens, laundry and toilets. For drinking, check your council’s regulations first — filtration, regular cleaning and maintenance are usually required.
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