Rainwater Tank Sizing Calculator
Sizing a rainwater tank means choosing enough storage to bridge the dry spells between rain. The simplest approach is demand-based: how much water you use per day, times the number of dry days you want the tank to cover, plus a margin for unusable volume and variability. This calculator returns the storage you need in litres and cubic metres. Every input is editable so you can match your demand and local rainfall records. Pair it with a roof collection estimate to confirm the tank can refill.
Rainwater tank sizing formula
Base storage = daily demand * dry days
Recommended tank = base storage * (1 + margin / 100)
Cubic metres = litres / 1,000
US gallons = litres / 3.78541
The base storage is the raw volume to cover your chosen dry spell. The margin adds headroom for unusable bottom volume and demand variability. The gallon conversion uses the exact 3.78541 litres per US gallon.
Designing the full system
- Match the dry-day figure to your local rainfall record, not a guess.
- Check that roof catchment area and rainfall can refill the tank between dry spells.
- Larger tanks cost more and take more space; size to the demand you actually offset.
- For potable use, follow your local health authority's treatment and first-flush rules.
Rainwater tank sizing: frequently asked questions
How do I size a rainwater tank?
A simple demand-based method multiplies your daily water demand by the number of dry days you want the tank to cover, then adds a safety margin. This gives the minimum useful storage volume. This calculator does that and reports the result in litres and cubic metres. A full design also checks that your roof catchment and local rainfall can refill the tank.
How many dry days should I plan for?
It depends on your climate and how critical the supply is. A garden-only tank might cover a week or two; a tank used for indoor non-potable supply in a dry region may need to cover a month or more of low rainfall. The dry-day figure is a user-editable input so you can match local rainfall records from your meteorological agency.
Why add a safety margin?
Tanks are rarely drawn fully empty, sludge and inlet height reduce usable volume, and demand varies. A margin (for example 20%) gives headroom so the tank does not run dry at the design point. The margin percentage is an editable input.
Does this account for how fast the tank refills?
No. This is a demand-side sizing estimate: it tells you how much storage covers a chosen dry spell. To confirm the tank can actually refill, check your roof catchment area and local rainfall with a roof rainwater collection calculator.
Official sources
- U.S. Environmental Protection Agency: Soak Up the Rain: Rain Barrels.
- U.S. National Oceanic and Atmospheric Administration: National Weather Service rainfall data.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.