Rainwater Tank and Cistern Size Calculator

Measure each connected roof section flat, as it would appear on a site plan, not up the slope: rain falls vertically, so a steep roof collects no more than its footprint. Leave out any section whose downpipes go to the stormwater drain or a soakaway, since none of its water reaches the tank. Sydney Water suggests a tank of at least 2,000 litres (528 US gal) connected to 75% or more of the roof as the minimum to consider, and says that getting as much roof as possible to flow to the tank does more for the savings than the tank's size.

Tools needed: Tape measure, Site plan or roof plan

The figure the page opens on is an example only, not anybody's rainfall, so replace it first. The Bureau of Meteorology publishes climate statistics for each Australian station with its mean annual rainfall; the Met Office gives 1991–2020 averages for UK climate stations; NOAA's U.S. Climate Normals give 1991–2020 station normals. Take the nearest station at a similar height, because rainfall changes quickly near hills and coasts. An average is the middle of a spread, and the dry year below it is the year the storage has to carry.

Approved Document G's rainwater calculation, the BS 8515 intermediate approach, multiplies area by rainfall by a 'yield coefficient and hydraulic filter efficiency': two factors, the share of the rain the roof sheds into its gutters and the share the filter passes on to the tank, which it takes together and gives one example for, 0.7. The page opens there, and this site's rainwater guides work to the same 0.7. Some of every shower is lost before the tank: the roof surface wets and dries off, water splashes past the gutter or evaporates, the first flush is diverted to waste and a filter rejects some. Where your filter or tank system's maker publishes a figure, use it. 1.0 would mean every drop arrives, which never happens.

Count the people living in the house, not the bedrooms. Approved Document G assumes two people in the first bedroom and one in each extra bedroom when it works out a new home's water use, which is a planning convention; the household as it actually lives is the better figure here.

Approved Document G's water calculator counts 1.46 full and 2.96 part flushes of a dual-flush WC a person a day, and 2.1 kg (4.6 lb) of washing. With the 6/4 litre (1.6/1.1 US gal) WC and the washing machine using 8.17 litres a kilogram (0.98 US gal a pound) in its Table 2.1, which is the most that table allows each fitting to use, that is about 20.6 litres (5.4 US gal) for the WC and 17.2 litres (4.5 US gal) for the washing, 37.8 litres (10.0 US gal) in all, rounded here to 38 litres (10.0 US gal). Thriftier fittings use less. Add a garden or anything else the tank feeds. For scale, the Australian Bureau of Statistics puts mains water use at 174 kL (46,000 US gal) a household in 2023–24, about 477 litres (126 US gal) a day for the whole household, and the US EPA puts home use at 82 US gallons (310 litres) a person a day.

The page opens on 18 days, the store this site's guide to sizing a rainwater harvesting tank works to: about five per cent of a year of the smaller flow (five per cent is 18.25 days), from the intermediate approach of BS 8515. That is a British standard, and its 18 days were set for UK rainfall, which falls fairly evenly through the year; where the dry season runs longer, as in much of Australia and parts of the US, the tank has longer to carry the house. BS 8515 was withdrawn in 2018 and replaced by BS EN 16941-1, whose basic approach BRE gives as the equivalent of the intermediate one. A dry season longer than 18 days is a reason to store more, as long as the roof can refill it; the store is never sized on more than the roof delivers, because a tank bigger than its roof can fill holds water that sits for months rather than flows. A month of autonomy on the household's use alone is the request that guide warns against.