A rainwater tank should be sized around the water you want to use, the rainfall your roof can collect and the gaps between useful rain events. Roof size alone cannot tell you which tank to buy.
Start by defining the tank’s job. Supporting vegetable beds through a dry fortnight is a different task from supplying toilets throughout the year. A separate emergency reserve has another purpose again.
Start with the demand you want to cover
Estimate how much water the proposed system will use each day. Include only the uses you intend to supply from the tank.
For garden irrigation, that might mean selected beds and containers rather than the entire garden. For household use, it might mean toilet flushing and other permitted non-potable uses rather than total household consumption.
Then choose a period you want the stored water to cover without useful new rainfall:
Daily target demand × days without useful inflow = first-pass usable storage
For example, a garden using an estimated 30 US gallons per day would need:
- 210 gallons for seven days
- 420 gallons for fourteen days
- 630 gallons for twenty-one days
The same calculation works in metric units. A target demand of 120 liters per day requires 1,680 liters for fourteen days.
These are illustrative demand figures, not recommended household or garden allowances. Use your own estimates.
They also assume that the required usable volume is available at the start. A partly full tank cannot provide the same coverage.
Check whether your roof can supply it
Storage holds water; it does not produce it.
Before choosing a capacity, estimate realistic roof collection using your connected catchment area, local rainfall and collection losses. Our guide to how much rainwater you can collect from your roof explains that calculation.
Compare supply and demand across the year. An annual collection total can look adequate while hiding a shortage during the months when you need water most.
This is particularly relevant to gardens: irrigation demand may rise during warm, dry weather, just as useful rainfall becomes less frequent.
Follow the water through the seasons
A monthly water balance provides a better planning check than an annual total alone. For each month, record starting storage, estimated collection and expected use. Track shortages and overflow as well as the amount carried into the next month. The Texas Water Development Board describes this approach in its rainwater-harvesting guidance.
Try several tank capacities using the same assumptions. Does extra storage prevent shortages by retaining water that would otherwise overflow? Or does the larger tank remain mostly empty because collection is too low?
Monthly figures are a useful first check, but they can conceal dry spells within a month. Systems expected to provide dependable household supply need closer examination of rainfall timing, dry periods and backup arrangements.
Bigger is not automatically better
Extra capacity helps when there is surplus water to store and a later demand for it.
If your catchment cannot supply enough water, a larger tank may add cost without delivering much additional usable water. Reducing demand, connecting more suitable catchment or retaining a backup source may be more effective.
Compare practical constraints too: available space, foundation requirements, installation access, overflow routing and maintenance access.
Check the manufacturer’s usable-volume information. The advertised capacity is not necessarily the volume available to your outlet or pump.
Choose a capacity you can justify
Before buying, you should be able to explain:
- Which uses the tank will supply
- How you estimated demand
- What dry period you are planning for
- Whether rainfall can replenish the storage
- What happens when the tank runs low
Tank sizing does not establish whether the water is suitable for a particular use. Water quality, treatment and plumbing arrangements require their own checks.
The free Self-Sufficient Water Planning Pack supports the planning process. The Self-Sufficient Water Handbook explains how storage fits into the complete system, alongside capture, pumps, treatment, distribution and backup.