How Big a Water Tank Do I Need?
A simple way to size a rainwater tank for the house, garden and animals, so you're not caught short when the rain stops.
If you live on tank water, the question isn't really "how big a tank?". It's "how long does my water have to last with no rain coming in?". Once you know that, the sum is short.
1. The short answer
Tank size ≈ the water you use each day × the number of days it has to last without rain, plus a safety margin.
Everything else in this guide is about getting those three numbers close to right.
2. Step 1: work out what you use each day
Add up three things.
The house. Your water bill, or a meter on the tank outlet, tells you your real use. If you don't have either, 150 litres per person per day is a common planning starting figure for use inside the house. Some households use a lot less, and some use more. It's a place to start, not a target.
The garden and other jobs. Watering, washing down the ute, a tap in the shed. Leave it out if the garden runs from a separate supply such as a dam or bore.
Animals. Chooks, dogs, a few sheep, a horse. How much each animal drinks changes a lot with hot weather, dry feed, and whether it's milking. Use your state agriculture department's figures for your animals rather than a guess, and use the hot-weather figure if there is one.
Write each one down in litres per day and add them up.
3. Step 2: decide how long it has to last
This is the dry spell: the number of days the tank must carry you with nothing coming in. For example, 60 days for a dry summer.
Look back at your own place. How long have you gone without useful rain in a bad year? Rainfall records for your town are on the Bureau of Meteorology's climate statistics pages (bom.gov.au). Plan for a dry run, not an average one.
4. Step 3: add a safety margin
Add something extra for:
- hot spells, when everyone (and every animal) drinks more
- visitors
- leaks and dripping taps
- the water that sits below the outlet, which you can't draw out.
The calculator starts at 20%. Use more if you have no other water source at all.
5. Worked example
A family of four, with six chooks and two dogs, wants to get through a 60-day dry spell. These are the calculator's starting figures.
| Litres per day | |
|---|---|
| House: 4 people × 150 L | 600 |
| Chooks: 6 × 0.3 L (example figure) | 1.8 |
| Dogs: 2 × 2 L (example figure) | 4 |
| Total | 605.8 |
- Use over 60 days: 605.8 × 60 = 36,348 litres.
- With a 20% margin: 36,348 × 1.2 = 43,617.6, so about 43,600 litres (43.6 kL).
- In 22,500-litre tanks: 43,617.6 ÷ 22,500 = 1.94, which rounds up to 2 tanks (45,000 litres).
The chook and dog figures are the calculator's rough examples only. Check your own animals' figures.
6. Can your roof actually fill it?
A big tank only helps if the roof can fill it. Every 1 mm of rain on 1 m² of roof is 1 litre of water, before losses. A 43,600-litre tank needs a lot of roof and a decent fall of rain to fill.
Our guide How Much Rainwater Can My Roof Catch? and the Roof Rainwater Catch Calculator work that out.
7. Before you buy
- Check the real capacity. Makers sell tanks by a nominal size. Check the actual litres on the spec sheet.
- Weight. Water weighs about a kilogram per litre, so a full 22,500-litre tank holds about 22.5 tonnes of water. It needs a level, properly prepared base. Follow the tank maker's instructions for the base.
- Drinking water. Keep the roof, gutters and tank clean and fit a first-flush diverter. Ask your local council or state health department about treatment.
- Approvals. Some councils have rules on tank size, placement or plumbing connections. Check before you install.
- Pumping it out. If the water has to go uphill or to a pressure tap, see What Size Water Pump Do I Need?.
8. Common mistakes
- Planning on an average year instead of a dry one.
- Forgetting the animals, or using cool-weather drinking figures.
- Counting the full tank as usable, when some water sits below the outlet.
- Buying a tank the roof can never fill.
- Not allowing for the base under a big tank.
9. Quick summary
- Tank size ≈ daily use × days without rain × (1 + margin).
- Get your real daily use from the bill or a meter if you can.
- Plan for a dry spell, not an average year.
- Check your roof can fill the tank.
- A full tank is very heavy, so it needs a proper base.
Try it with your own figures: Water Tank Size Calculator.
General information only. Check the tank maker's instructions, and ask your council about approvals and drinking water.