Rainwater Harvesting Calculator
Estimate how much rainwater you could collect from your roof — for the garden, or to cut mains water use. Based on your roof's footprint area and local rainfall, with a realistic collection efficiency.
How to use this calculator
- Enter your roof collection area — the flat footprint, since rain falls vertically.
- Enter your local annual rainfall in mm.
- Pick your roof type for a realistic collection efficiency.
Formula used
The neat fact underpinning this: 1 mm of rain on 1 m² of area is exactly 1 litre. Multiply roof footprint by annual rainfall for the theoretical maximum, then apply an efficiency factor for the roof surface, first-flush diversion and overflow losses.
Example calculation
An 80 m² roof, 750 mm rainfall, pitched tile (85%): 80 × 750 × 0.85 = ~51,000 litres/year.
That's enough to fill a 200-litre water butt about 255 times — plenty for a garden.
How much rain a roof yields
The surprising thing about rainwater harvesting is how much water an ordinary roof collects, thanks to the tidy relationship that a millimetre of rain on a square metre is a litre. Even a modest roof in a moderately rainy climate sheds tens of thousands of litres a year — water that otherwise runs straight into the drains. Capturing it for the garden cuts mains use, and in dry spells it's free irrigation exactly when you need it.
Real-world collection falls short of the theoretical maximum for a few reasons the efficiency factor captures: some water evaporates or clings to the roof, first-flush diverters deliberately discard the initial dirty runoff, and any collected rain beyond your storage capacity simply overflows. The footprint area (not the sloped surface) is what counts, because rain falls vertically. For garden use it's straightforward; using harvested rain indoors or for drinking needs proper filtration and treatment.
Why use this calculator?
- See how much water your roof could actually collect.
- Size a water butt or tank to your roof and rainfall.
- Efficiency factors for different roof types built in.
Frequently asked questions
How much rainwater can I collect from my roof?
Multiply roof footprint area (m²) by annual rainfall (mm) by an efficiency factor (~0.85 for tile). An 80 m² roof in a 750 mm rainfall area collects roughly 51,000 litres a year — since 1 mm on 1 m² equals 1 litre.
Do I use the roof's sloped area or footprint?
The footprint (the flat area the roof covers when viewed from above), because rain falls vertically. A steeper roof doesn't collect more rain than its footprint would — the slope just directs the same rainfall to the gutters.
Can I drink harvested rainwater?
Not without proper treatment. Roof runoff picks up dust, debris and contaminants, so harvested rainwater is ideal for gardens, toilets and outdoor use, but needs filtration and disinfection before any drinking use.