Water Tank Sizing Calculator

To estimate how much rainwater you can collect from your roof, use the formula: roof catchment area in square metres multiplied by annual rainfall in millimetres, divided by 1,000, gives annual collection in litres. A practical collection efficiency factor of around 0.85 to 0.90 is applied to account for evaporation, first-flush losses, and overshoot. For example, a 150 m2 roof in Wellington (approximately 1,200 mm annual rainfall) could yield 150 x 1,200 x 0.85 / 1,000 = approximately 153,000 litres per year. A first-flush diverter, which discards the first few litres of rain that washes pollutants off the roof, significantly reduces contamination. Roof material also affects quality: coated metal roofs are generally preferred over untreated timber or asphalt shingles for drinking water collection. Source: NIWA regional rainfall data (niwa.co.nz); Building Performance (building.govt.nz).

Tank sizing in New Zealand depends on your water use, rainfall patterns, roof catchment area, and whether the tank is your sole or supplementary supply. For a household of 3 to 4 people using a tank as sole supply, 25,000 to 50,000 litres is a common recommendation. A 90-day storage buffer is often recommended in drier regions such as Canterbury and Hawke's Bay, where summer rainfall is low. The basic sizing formula is: daily household water use (litres) multiplied by the number of storage days required. Average NZ household water use is approximately 150 to 200 litres per person per day. For supplementary use such as garden watering, toilet flushing, or laundry, a 5,000 to 10,000 litre tank is usually sufficient. Your installer or a licensed plumber familiar with local rainfall data can help size your system accurately. Source: Building Performance clause G12 (building.govt.nz); NIWA (niwa.co.nz).

New Zealand has highly variable rainfall due to its mountainous terrain and position across the Tasman Sea weather systems. Key regional annual average rainfall figures from NIWA are approximately: Auckland 1,200 mm; Whangarei 1,600 mm; Hamilton 1,200 mm; Wellington 1,250 mm; Nelson 960 mm; Christchurch 640 mm; Dunedin 800 mm; Invercargill 1,040 mm; West Coast (Westland) 5,000 to 10,000 mm. Rainfall is highly seasonal in many regions: the east coast of both islands tends to be drier in summer, while the West Coast receives rain year-round. For tank sizing and rainwater harvesting planning, it is important to use monthly rainfall distributions rather than annual totals, as summer dry spells determine storage requirements. NIWA provides free regional climate data through its CliFlo database. Source: NIWA (niwa.co.nz) regional climate data.

Yes, rainwater collection systems in New Zealand are covered by the Building Code, specifically Clause G12 (Water Supplies), which sets performance requirements for water supply systems used for human consumption. Water used for drinking must meet the New Zealand Drinking Water Standards, which require that water be safe, acceptable, and fit for purpose. For tanks used as a sole supply for drinking water, a treatment system is typically required, such as a first-flush diverter, sediment filter, and ultraviolet (UV) disinfection unit. The tank, pipework, and fittings must be made from food-grade materials compliant with AS/NZS 4020 (testing of products in contact with drinking water). A licensed plumber must carry out the installation. Consent requirements vary by council, so check with your local council before installing a large tank. Source: Building Performance Clause G12 (building.govt.nz); Ministry of Health drinking water guidelines (health.govt.nz).