Rainwater tank calculator

The one number to remember: 1 mm = 1 L per m²

Rainfall is measured as a depth. A millimetre of rain over one square metre is a layer of water 1 mm deep, which is one litre. Everything else in the calculator follows from that:

Use the roof's footprint, not its surface. Rain falls vertically, so a steep roof catches no more than a flat one with the same footprint. Measure the building from outside, include the eaves, and count only the roof sections that drain to the tank.

Why 80 %? A clean metal roof sheds 90–95 % of the rain that hits it; tiles a little less. A first-flush diverter then sends the dirty first few millimetres of each storm to waste, and heavy downpours overflow gutters and a full tank. Together that usually leaves 75–85 % of the theoretical figure. Use 85 % for a metal roof with clean gutters and 70–75 % for an old tile roof or a small tank that often overflows.

Worked example

An 80 m² section of metal roof in an area with 600 mm of rain a year, feeding a 5,000 L tank that waters a 20 m² vegetable garden:

1. Collected per millimetre: 80 × 0.8 = 64 L. Per year: 64 × 600 = 38,400 L.
2. A 10 mm shower adds 640 L. From empty, the tank fills after 5,000 ÷ 64 = 78 mm of rain.
3. Watering 20 mm a week in spring uses 20 × 20 = 400 L a week, so a full tank lasts 12.5 weeks without rain.
4. In a midsummer heatwave the same garden may need 35 mm a week (700 L). The full tank now lasts about 7 weeks.

The roof collects enough in a year for 96 weeks of spring watering. The tank is the bottleneck, not the roof.

Average rainfall by city

These long-term averages show the range between weather stations in the same city. In Sydney, for example, the airport records noticeably less than the city centre. Always prefer the figure from your nearest station.

The seasonal column matters as much as the total. In Perth and Adelaide most rain falls in winter, when the garden needs the least water, and the tank has to carry the garden through a long dry summer. In Brisbane and Darwin the rain arrives in the hot season, so a smaller tank refills far more often.

Choosing a tank size for the garden

Size the tank for the longest dry spell you want to bridge, not for the year's total:

Bridging six dry weeks for a 20 m² vegetable garden at 30 mm a week needs 20 × 30 × 6 = 3,600 L, so a 4,000–5,000 L tank. Doing the same for a 100 m² lawn needs 18,000 L. That is why lawns are usually left on mains water or allowed to go dormant, and tanks go to the vegetable garden and fruit trees.

How much water a garden needs depends on the weather. Summer evaporation in the southern capitals commonly runs at 5–8 mm a day, so a mulched vegetable bed in full sun can use 30–40 mm a week or more in a hot spell, and well under half that in spring and autumn. Established native gardens often need little or nothing once rooted. Mulch cuts the figure substantially; see the mulch calculator.

Getting more out of a tank

• Drip irrigation under mulch puts nearly all the water where the roots are. Sprinklers lose a large share to wind and evaporation on a hot day.
• Gutter guards and a first-flush diverter keep leaves and roof dust out, so the water stays clear and the pump and drippers don't clog.
• Insect-proof screens on every inlet and overflow keep mosquitoes out. Health authorities recommend them, and some states make them compulsory.
• Check local rules. Some councils and water utilities set requirements or offer rebates for tanks; ask before you buy. If you plan to drink tank water, follow the Australian Government's enHealth guidance on rainwater tanks. This page is about the garden only.

Rainwater questions