Roof Water Catchment Calculator NZ 2026
This calculator works out how much rainwater your roof can collect over a year, a month and a day, which is the starting point for sizing a rainwater tank or planning an off-grid or supplementary water supply in New Zealand. The principle is simple and powerful: every millimetre of rain that falls on a square metre of roof delivers one litre of water, so a surprisingly large amount runs off even a modest roof in a country as wet as New Zealand. The catch many people miss is that it is the roof footprint, the area as seen from above, that matters, not the sloped surface area, because the rain falls vertically. You also lose some water to the first flush that washes the roof clean, to splashing and overflow in heavy downpours, and to evaporation, which is why a collection efficiency of around 75 to 90 percent is realistic rather than the full theoretical amount. You enter your roof footprint in square metres, your local average annual rainfall in millimetres, which ranges from around 600mm in the driest parts of the country to well over 2,000mm on the West Coast, and a collection efficiency, and the calculator returns the litres you could collect each year, the average per month, and the average per day. Use it to size a tank, to see whether your roof can meet garden, laundry or whole-house demand, or to estimate how much town or bore water rainwater harvesting could offset. Rainfall is seasonal and uneven, so a tank must bridge dry spells; pair this with a tank-size calculator. Treat the result as a planning estimate based on average rainfall.
Uses roof footprint (plan area), not sloped area. Rainfall is seasonal, so size a tank to bridge dry spells. An estimate from average rainfall.
How it works
One millimetre of rain on one square metre of roof gives one litre of water. The annual collection is the roof footprint times the annual rainfall times the collection efficiency. That yearly figure is divided by 12 for the average month and by 365 for the average day.
Worked example
A 120 square metre roof footprint in an area with 1,200mm of annual rainfall, at 80 percent collection efficiency, collects about 115,200 litres a year, or 115.2 cubic metres. That averages around 9,600 litres a month and about 316 litres a day, though the real supply is much higher in winter and lower in summer.
Related calculators
- Rainwater Tank Size: size your tank.
- Water Usage Cost: the cost of water.
- Pipe Volume & Flow: pipe capacity.
- Spouting Length: guttering to collect it.