Formula
Water to store = people × use per person per day × days of reserve. The calculator then suggests the next common tank size.
Example
4 people using 80 gallons each per day use 320 gallons a day. Two days of reserve need 640 gallons, so a 750-gallon tank fits with room to spare.
How much water does a person use?
- In US homes, the average is around 80 gallons per person per day indoors.
- Emergency reserves: at least 1 gallon per person per day for drinking and basic hygiene, for several days.
- Gardens and pools use much more in summer.
Frequently asked questions
How many days of reserve should I keep?
For a home on a public supply, 1 to 2 days covers most outages. Homes on wells or rainwater often keep more.
Can I use two smaller tanks?
Yes. Two connected tanks are often easier to move, install and clean than one large tank.
See also
Results are estimates for information only and are not financial, legal or medical advice. Rates, taxes and fees vary — confirm with a professional or your lender.