How to size a rainwater tank for your roof
One millimetre of rain falling on one square metre of roof is one litre of water.
That rule makes tank sizing a few minutes' arithmetic. What makes it tricky is that rain doesn't arrive evenly, so the right tank is the one that stores wet-season water for the dry months, not the one that holds a year's rain.
Step by step
Use the roof's plan area (its footprint, not the sloped surface) that drains to the tank's downpipes.
The Bureau of Meteorology publishes long-term monthly averages for weather stations near you.
Use about 80% efficiency to account for first-flush diverters, evaporation and overflow in heavy storms.
Garden watering, toilets and the laundry are typical tank uses. Check the monthly gap in your driest months.
Worked example
| Item | Calculation | Result |
|---|---|---|
| Yearly harvest | 150 m² × 600 mm × 0.8 | 72,000 L |
| Monthly average | 72,000 ÷ 12 | 6,000 L |
| Tank, 2.2 m diameter × 2.0 m high | π × 1.1² × 2.0 × 1,000 | ≈ 7,600 L |
| Supply at 200 L a day | 7,600 ÷ 200 | ≈ 38 days |
✨ Key Takeaways
- Litres = roof m² × rainfall mm × efficiency.
- Size the tank for your dry season, not your yearly total.
- Use a licensed plumber for indoor connections.
Open the rainwater tank calculator →