Rainwater Harvesting & Catchment Calculator
Work out how many litres your roof can genuinely collect per year, using real long-term Australian rainfall averages, and get a realistic tank size recommendation.
The maths behind rainwater harvesting is genuinely simple once you know it: one millimetre of rain falling on one square metre of roof produces exactly one litre of water, before losses. Everything else in this calculator is just accounting for those losses accurately.
Catchment Calculator
This uses real long-term Bureau of Meteorology rainfall averages for each capital, as a starting reference — your specific location may differ from the nearest capital, sometimes significantly, so treat this as a planning estimate.
Estimated annual harvest: —
Roughly per month (average): —
Suggested minimum tank size: —
Why the Runoff Coefficient Matters
Not every litre that falls on your roof reaches the tank. Some evaporates, some is absorbed by the roof surface itself, and some is lost to splash and gutter overflow in heavy downpours. Metal roofing (the most common choice on new Australian sheds and homes, and already the material recommended throughout this site's own build guides) is the most efficient catchment surface at roughly 0.85-0.95, since it's smooth and non-porous. Tile sits a little lower at 0.75-0.85, and concrete lower again at 0.6-0.7, since both retain more moisture on the surface before it runs off. A first-flush diverter, which discards the first 1-2mm of each rain event (carrying roof dust, leaves, and bird droppings), further improves water quality without meaningfully reducing the volume you collect over a full year.
Sizing Your Tank From This Number
A common, sensible approach is to size storage around roughly six weeks of average collection, rather than trying to capture an entire year's rainfall in one tank — that's rarely practical or necessary, since a home or homestead is drawing down the tank continuously, not just once a year. For a genuinely productive homestead with a full garden and animals, the wider planning benchmark used elsewhere on this site is 90,000–120,000L of total catchment capacity with proper multi-stage filtration, though the right number for your specific setup depends heavily on your actual roof area, regional rainfall, and how much of your water use you want rainwater to cover.
Frequently Asked Questions
Does roof pitch (steepness) affect how much water I collect?
No. Rain falls vertically, so the horizontal plan (footprint) area of your roof is what matters for collection volume, not the sloped surface area. A steeper roof has more physical surface but intercepts exactly the same rainfall as a flatter roof with the same footprint.
What roof material collects the most rainwater?
Metal/Colorbond roofing has the highest runoff coefficient (roughly 0.85-0.95) of common roofing materials, making it the most efficient choice for rainwater harvesting as well as being the standard recommendation for homestead structures generally.
How much rainwater tank storage do I actually need?
A common approach is sizing for around six weeks of average collection. For a full homestead setup with garden and animals, 90,000-120,000L of total capacity is a reasonable wider planning benchmark, though your specific roof area and regional rainfall will change the ideal number.
Is this calculator using my exact local rainfall?
It uses long-term averages for the nearest capital city as a starting reference. Your specific location may genuinely differ, sometimes significantly — use the custom rainfall option if you know your local average from the Bureau of Meteorology.
For the rest of your homestead water planning, see our Water how-to guides.