Water Tank Size Calculator
Sizing Storage Around Actual Daily Demand
A storage tank — whether it's a cistern, a well-system pressure reserve, or an off-grid holding tank — only works if it holds enough water to cover consumption between refills. That means the sizing question isn't really about the tank at all; it's about how much water the people relying on it actually use in a day, multiplied by how many days of independence the tank needs to provide.
The Formula
Tank Size = Daily Demand × Storage Reserve (days)
The default of 80 gallons per person per day reflects typical U.S. household per-capita indoor water use figures published by water utilities and the EPA, covering showers, toilets, laundry, and kitchen use combined.
Where This Calculation Matters
- Rural and well-water properties — sizing a pressure or storage tank to buffer against well recovery rate limitations.
- Off-grid and cabin systems — determining how many days a cistern will last between refill deliveries.
- Rainwater harvesting — matching storage capacity to household demand during dry stretches between rain events.
- Emergency preparedness — calculating minimum storage to cover a multi-day outage of the primary water supply.
Typical Per-Person Daily Water Use
| Fixture | Typical daily use |
|---|---|
| Showers | ~17 gal |
| Toilets | ~18 gal |
| Faucets | ~15 gal |
| Clothes washer | ~15 gal |
| Leaks and other use | ~15 gal |
| Typical total | ~80 gal |
These are widely cited average U.S. indoor per-capita figures; actual use varies by fixture efficiency, household habits, and climate — adjust the gallons-per-person input if you have metered data for your household.
How to Use This Calculator
- Enter the number of people the tank needs to serve.
- Optionally adjust gallons per person per day (defaults to 80).
- Optionally adjust the storage reserve in days (defaults to 1).
- Select Calculate to get the recommended tank size in gallons.
Related Calculations
For sizing the water heater feeding that same household, see the Water Heater Size Calculator. To size the septic system handling the resulting wastewater, use the Septic Tank Size Calculator.