Start with demand
Most people guess. Most people guess low. Then they buy a tank half the size they needed.
Do the accounting first.
Garden. A vegetable bed in warm weather drinks 4 to 6 litres per square metre per day. A 50 m² kitchen garden runs 200 to 300 litres a day, or 6,000 to 9,000 litres a month.
Trees. Figure 40 to 80 litres per established fruit tree per week. Ten trees add 400 to 800 litres weekly.
Lawn. If you water it: 4 to 8 litres per square metre per week. Most years, don't.
A productive property with garden, small orchard, and a few ornamentals burns 15,000 to 25,000 litres a month in season.
Household. Pipe rainwater to toilets and laundry and you add real load. Toilet flushing: 30 to 50 litres per person per day. A family of four uses 5,000 to 8,000 litres a month for toilets and washing alone.
The number that matters is the longest stretch you expect to run on stored water without meaningful rain.
Calculating supply
Supply is roof area times rainfall times runoff coefficient. Use 0.80 to 0.85 for metal roofs after gutter losses and the first-flush diverter. See the catchment calculation page for the long version.
Annual rainfall is the wrong number to size on. Monthly pattern is the right one.
800 mm spread evenly across the year means a small tank refilling continuously. The same 800 mm dumped in a 4-month wet season means a tank that must bridge 8 dry months. Same rainfall, completely different storage.
Build the monthly budget
Put twelve rows in a spreadsheet. For each month, enter expected supply (roof × rainfall × coefficient) and expected demand. Start the dry season with a full tank and run the cumulative balance.
The deepest deficit is your minimum tank size.
Worked example. A 100 m² metal roof in a climate with a 5-month dry season. Demand runs at 8,000 litres a month. Supply during the dry months drops to 1,000. Cumulative deficit: 35,000 litres.
That is the volume you need. Not a guess. A number.
Matching the two sides
The budget tends to expose one of two problems.
Supply beats demand on the year, but timing fails. 60,000 litres of roof catchment against 30,000 litres of use looks comfortable. If 50,000 of those litres arrive in 4 months and use is spread across 12, you still need a big tank to bridge the gap.
Demand beats supply. No tank fixes this. Cut demand with drip irrigation, mulch for moisture, and wicking beds. Or grow supply by connecting more roof, adding surface catchment, or topping up from another source.
Find the bottleneck before you buy plastic.
Choosing the actual tank
One big tank is cheapest per litre stored. A 10,000-litre poly tank runs 30 to 50 percent less per litre than two 5,000s.
Multiple smaller tanks buy redundancy. One leaks or needs cleaning, the others keep flowing. They squeeze into spaces a single giant won't, and you can split them across different elevations to gravity-feed different zones.
Scale crushes the unit cost. A 1,000-litre tank: 0.50 to 1.00 per litre. A 10,000-litre tank: 0.10 to 0.20. A 25,000-litre tank or underground cistern: under 0.10. Ferrocement tanks poured on site, where labour is cheap, can drop below 0.05.
Buy the minimum the budget tells you to. Plan the next one. Most systems extend cleanly by tying a second tank in at the base for equalised levels, or off the overflow for cascading fill.
A 5,000-litre tank now plus a second next year usually beats a 10,000-litre tank that wrecks the budget for the soil organic matter work that cuts demand in the first place.
See also
- Roof catchment calculation for the supply number
- Tank placement for shade, gravity, and overflow geometry
- Overflow management so excess water builds soil instead of cutting gullies
- Drip irrigation to shrink the demand side
- Soil water storage the biggest tank on the property