Rainwater Collection Calculator
See how many litres of rainwater your roof could harvest — useful for sizing a rainwater tank or understanding how much water your gutters handle in a storm.
Estimated collection
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Enter your roof area and a rainfall amount to see the estimate.
Results are estimates for planning purposes only. Every roof is different — for exact measurements and a fixed-price quote, book a free on-site inspection or call 0416 689 689.
How the calculation works
The maths behind rainwater harvesting is beautifully simple: one millimetre of rain falling on one square metre of roof equals one litre of water. Multiply your roof's catchment area by the rainfall in millimetres, then allow a little for losses — water that wets the roof surface, splashes over gutters or is diverted by a first-flush device — and you have a realistic collection estimate.
Don't know your roof area? Estimate it in seconds with our roof area calculator. For tank sizing, try the calculation with your area's typical monthly rainfall to see what a wet month could deliver.
Your gutters do the heavy lifting
The same numbers explain why gutters and downpipes matter so much. In a heavy Sydney downpour, even a modest roof sheds thousands of litres — all of it funnelled through your guttering. Blocked, sagging or undersized gutters overflow into eaves and walls long before the roof itself leaks.
If your gutters overflow in heavy rain or you're planning a rainwater tank, our team can assess your roof drainage as part of a free inspection.
Ready To Protect Your Home?
Book Your Free Roof Inspection Today.
Whether you need emergency roofing, leak detection, storm damage repairs or simply peace of mind, our experienced roofing specialists are ready to help. No pressure. No hidden costs. Just honest roofing advice.