This New Zealand calculator helps homeowners and lifestyle block owners size a rainwater tank for their needs, whether for a rural water supply, garden use, or supplementing town supply. You enter your roof catchment area, the local rainfall, and your expected water use, and the tool estimates how much water you could collect and what tank size would suit, balancing collection against demand through dry periods. The breakdown shows the potential collection, your usage, and the recommended capacity, helping you choose a tank that neither runs dry in summer nor wastes money on unnecessary size, with local rainfall used as an input so it works anywhere in the country.
Collection is about one litre per square metre per millimetre of rain, reduced by the runoff coefficient for losses. The suggested capacity covers your use across the dry spell you enter. Rainfall is seasonal, so a larger tank gives more buffer. Estimate only, check local rules for drinking water.
Your roof can collect about one litre of water for every square metre of area and every millimetre of rain, so the annual collection is your area times rainfall times the runoff coefficient, which allows for losses. Your annual use is your daily use times 365. The suggested tank size is your daily use times the dry-spell days, so the tank can carry you through a period without meaningful rain. Comparing annual collection with annual use shows whether your roof can keep up with demand over the year.
A 150 square metre roof in a 1,200 mm rainfall area at a 0.8 runoff collects about 144,000 litres a year. Using 500 litres a day is 182,500 litres a year, so this roof alone would not fully supply that use. To bridge a 30 day dry spell, a tank of about 15,000 litres is suggested.
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