Rainwater Tank Size Calculator

This calculator helps you size a rainwater tank for a home or lifestyle block anywhere in New Zealand, whether you want an off-grid rural supply, need to top up town water, or just want tank water for the garden and washing. You enter your roof catchment area in square metres, your local annual rainfall in millimetres, a runoff coefficient allowing for losses from gutters and first flush, where 0.8 is a common starting point, your expected daily water use in litres, and how many dry-spell days you want the tank to cover without meaningful rain. From these figures the calculator returns your suggested tank capacity, your estimated annual collection from the roof, your annual water use, and a comparison showing whether your roof can realistically keep up with demand over a full year. Collection is worked out at roughly one litre per square metre of roof for every millimetre of rain, adjusted for the runoff coefficient, while the recommended tank size is your daily use multiplied by the dry-spell length you specify. Because rainfall varies through the year and between regions, a bigger tank buys you more buffer against a run of dry weeks. Treat the figures as a planning estimate to compare tank sizes and suppliers, and check your local council's rules if you plan to rely on tank water for drinking.

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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.