Estimate how much rainwater a roof can collect over a year from its area, your local rainfall and a runoff allowance — the potential yield to inform tank sizing.
This is potential collection, not tank size. The right tank balances collection against demand and how rainfall is spread across the year — a big roof in a dry month still won’t fill an oversized tank.
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Start free trialOne millimetre of rain on one square metre of roof is one litre of water, so roof area times rainfall gives the gross catch.
Not all rain is captured — first-flush diversion, evaporation, splash and overflow lose some. A runoff coefficient of around 80–90% allows for that.
The annual figure divided into monthly and daily averages helps you judge supply against demand — though real rainfall is far from even.
Multiply roof area (m²) by annual rainfall (mm) by a runoff factor. A 150 m² roof at 600 mm rainfall and 85% runoff collects about 76,500 litres a year. Because 1 mm on 1 m² equals 1 litre, the maths is simple — the hard part is that rainfall isn’t evenly spread.
It’s the fraction of rain that actually ends up in the tank after losses. First-flush diverters, evaporation off a hot roof, splash and overflow all reduce the catch, so a hard roof typically collects 80–90% of the rain that falls on it.
Tank size balances how much you collect against how much you use and how evenly the rain falls. A large roof in a climate with a long dry season needs a bigger tank to carry water through the dry months; a smaller tank suits steady rainfall and modest demand. This tool shows potential collection to inform that choice.
Rainwater connected to household use (like toilets or laundry) must be installed to the plumbing code, often with backflow prevention and correct signage, by a licensed plumber. Requirements vary by state and council, so check locally.
This calculator estimates potential rainwater collection, not tank size or yield reliability. Actual yield depends on rainfall distribution, demand and system losses. Rainwater connections must comply with AS/NZS 3500 and local requirements and be installed by a licensed plumber.
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