Store at least 1 gallon of water per person for every day you want to cover. That is the figure from Ready.gov and the CDC, and the calculator above multiplies it for you. For a family of 4 and the 3 days the CDC starts from, the answer is 12 gallons. For 2 weeks, which the CDC suggests aiming for "if possible", the same family needs 56 gallons.

What the numbers are based on

Ready.gov says to store at least one gallon of water per person per day for several days, for drinking and sanitation. The CDC puts the same rule more exactly: 1 gallon per person, per day, for 3 days, and a 2-week supply if you can manage it.

The gallon is not all drinking water. Ready.gov notes that a normally active person needs about three quarters of a gallon of fluid daily, from water and other beverages. The rest of the gallon covers things like brushing teeth and washing hands, so cutting the amount to what you can drink leaves you with nothing to clean with.

The calculator uses the official figure as is. It does not add a safety margin of its own, and it does not guess at your climate or health. Those adjustments are yours to make, and the next section lists the ones the agencies name.

Who needs more than 1 gallon

Ready.gov says children, nursing mothers and sick people may need more water, and that in very hot temperatures water needs can double. The CDC adds pregnant women and pets to the list. The UK government says extra water might be needed to make up baby formula and for medical devices as well.

None of the three gives a number for pets or for any of these groups, so we don't either. If you have a baby, a dog or someone who relies on a medical device, round up and ask your own doctor or vet what they need. The "extra" note under the calculator is a reminder, not a calculation.

If you use litres

The UK government recommends 3 litres of drinking water per person per day for 3 days. That figure is for drinking only, so it is not a like-for-like swap with the US gallon, which also has to cover sanitation. A US gallon is 3.785 litres, according to the National Institute of Standards and Technology. A UK household that follows the 1 gallon rule ends up with a bit more than the UK recommends, which is the safer side to be on.

Common mistakes

Storing only the drinking amount is the usual one. The gallon has to cover washing and brushing teeth too.

Old water is the second. Ready.gov and the CDC both say to replace water you filled yourself every 6 months. Write the fill date on the container. Commercial bottles carry their own expiry date.

Third, drinks are not a substitute. Ready.gov says not to drink carbonated or caffeinated beverages instead of drinking water.

Last, check the sum before an emergency, not during one. Add the people, multiply by the days, and look at how many containers that is and where they will sit. 12 gallons is a lot of floor space.

This page is reference material from official sources, not medical advice. If someone in your household is ill, pregnant or on medication, ask a doctor how much water they need.