The tank size is set by the dry run, not by the rain
Your roof decides whether the tank ever fills. The dry spell decides how big it has to be. Those are two different questions and most tank calculators only answer the first one.
A 150 m² Sydney roof catches around 154,000 litres a year. That sounds like plenty for a household using 73,000 litres a year, and it is, on the annual total. But rain does not arrive on a schedule. If your area goes two months without useful rain and you are drawing 200 litres a day, you need 12,000 litres sitting in the tank when the dry starts. A 5,000 litre tank fed by that same generous roof runs empty in twenty-five days.
So size the tank on the dry run, then check the roof can refill it.
Rainwater tank sizes, and what each one is good for
Round poly tanks run from about 1,000 to 46,400 litres, and for a house on mains the useful band is 2,000 to 20,000. Your Home, the federal government's guide to building, puts these targets on a 4 person household with mains water and rain spread through the year.
| Use | Target tank |
|---|---|
| Toilet flushing and use on a small garden | 2,000 to 3,000 L |
| Toilet flushing, clothes washing and small garden | 3,000 to 5,000 L |
| Whole of house water supply | 5,000 to 20,000 L |
Where the rain comes in one season, or there's no mains, it says much larger tanks are typically required. (Your Home, Rainwater, read 26 October 2026.)
And this is what those sizes are on the ground, from one maker's round poly range. The diameter is the pad you prepare, plus a margin, and every litre is a kilogram once it's full.
| Tank | Diameter × height | Water in it, full | Lasts at 200 L a day |
|---|---|---|---|
| 1,500 L | 1.19 × 1.62 m | 1.5 t | 7 days |
| 2,400 L squat | 1.81 × 1.20 m | 2.4 t | 12 days |
| 3,250 L tall | 1.57 × 2.00 m | 3.25 t | 16 days |
| 5,000 L tall | 1.83 × 2.20 m | 5 t | 25 days |
| 5,000 L squat | 2.06 × 1.86 m | 5 t | 25 days |
| 10,000 L tall | 2.57 × 2.44 m | 10 t | 50 days |
| 10,000 L squat | 3.08 × 1.81 m | 10 t | 50 days |
| 15,000 L | 3.08 × 2.48 m | 15 t | 75 days |
| 22,500 L | 3.78 × 2.56 m | 22.5 t | 112 days |
| 30,000 L | 3.95 × 3.08 m | 30 t | 150 days |
| 46,400 L | 4.60 × 3.16 m | 46.4 t | 232 days |
Sizes and dimensions are the Bushmans poly range as listed on its site (bushmantanks.com.au, read 26 October 2026). Other makers sell close steps, not the same ones, so check the one you're buying. The calculator above rounds up to this range. The same range has slimline tanks from 660 to 5,000 litres for a fence line or the side of the house. Round tanks are usually the cheapest per litre, Your Home says, and slimlines usually need a properly formed concrete slab, depending how tall they are against their width.
Long-term rainfall, by station
These are Bureau of Meteorology long-term means. The record length matters. A mean over 160 years is a much steadier number than one over 12.
| Station | Annual mean | Record |
|---|---|---|
| Cairns Aero (031011) | 2,025.5 mm | 83 years |
| Darwin Airport (014015) | 1,736.8 mm | 85 years |
| Coffs Harbour Airport (059151) | 1,727.8 mm | 12 years |
| Gold Coast Seaway (040764) | 1,302.2 mm | 31 years |
| Sydney Observatory Hill (066062) | 1,211.1 mm | 162 years |
| Townsville Aero (032040) | 1,162.3 mm | 85 years |
| Wollongong University (068228) | 1,148.5 mm | 27 years |
| Newcastle Nobbys (061055) | 1,124.6 mm | 161 years |
| Brisbane Airport (040842) | 1,030.9 mm | 31 years |
| Toowoomba Airport (041529) | 737.2 mm | 30 years |
| Perth Metro (009225) | 718.5 mm | 33 years |
| Ballarat Aerodrome (089002) | 683.1 mm | 118 years |
| Launceston Ti Tree Bend (091237) | 679.8 mm | 46 years |
| Melbourne Regional Office (086071) | 648.3 mm | 160 years |
| Canberra Airport (070351) | 635.5 mm | 17 years |
| Hobart Ellerslie Road (094029) | 611.0 mm | 144 years |
| Adelaide West Terrace (023000) | 524.5 mm | 149 years |
| Alice Springs Airport (015590) | 283.8 mm | 84 years |
Rainfall varies a lot over short distances, the Dandenongs against inner Melbourne, the escarpment against the Illawarra flats. If your suburb is wetter or drier than the nearest capital, look your own station up at the Bureau and type the number in.
Two worked examples
Sydney, 150 m² roof, 200 L a day, a two-month dry run.
- Catch, 150 × 1,211.1 × 0.85 = 154,415 L a year
- Use, 200 × 365 = 73,000 L a year, and the roof catches more than twice that
- Tank, 200 × 60 = 12,000 L, so the next size up in that range is 15,000 L
- Every 10 mm of rain puts 1,275 L in that tank
Adelaide, 80 m² roof, 150 L a day, a three-month dry run.
- Catch, 80 × 524.5 × 0.85 = 35,666 L a year
- Use, 150 × 365 = 54,750 L a year, and the roof only covers about 65% of it
- Tank, 150 × 90 = 13,500 L, so the next size up is 15,000 L
- The tank size is right and the roof still cannot keep up. Either connect more roof, use less, or accept topping up from the mains in a dry year.
What people get wrong about roof area
- Use the plan area, not the sloped area. Rain falls vertically, so a steep roof catches no more than a flat one of the same footprint. Measure the outline of the building, including the eaves.
- Only count what actually drains to the tank. Half a roof plumbed to the tank is half the catch, however big the house is.
- A shed or carport counts. It is often the cheapest way to add catchment, and the downpipe run is short.
The parts that are not about size
- Gutter guard and a first-flush diverter. The diverter throws away the first few millimetres carrying roof grit and bird mess. It is why the runoff coefficient is not 1.0.
- Overflow has to go somewhere legal. A tank overflow that dumps against a footing or over a boundary is a problem you build in once and live with.
- The base. A full 10,000 litre tank weighs about 10 tonnes. It needs a properly prepared pad, and on sloping or filled ground that is an engineering question, not a levelling job.
- Drinking water is a different conversation. Roof caught water for drinking has health guidance attached, roof material, flashing, lead paint, nearby stacks. Ask your state health department before you plumb a tank to a kitchen tap.
- Some councils require a tank on a new build or a major renovation, at a set minimum size and plumbed to set fixtures. Check before you buy, because the required size may be bigger than the useful one.
Other calculators you'll want
General information for Australian homeowners and trades, and an estimate only, not legal advice, and not planning or building advice, and not a substitute for a proper sizing by a plumber, tank supplier or engineer. Rainfall figures are long-term means read from the Bureau of Meteorology on 3 September 2026 and an average is not a forecast. A real drought runs longer than an average dry spell. The runoff coefficient is a rule of thumb, and no Australian Standard fixing it was found. Rainwater used for drinking carries state health guidance, tank supports and overflow can be engineering questions, and some councils set a minimum tank size and required connections. No regulator has reviewed this page.