How much water to store, and why one gallon
The planning rule is published and stable: FEMA and Ready.gov say to store at least one gallon of water per person per day. That gallon is not all for drinking - the guidance splits it roughly in half, about two quarts for drinking and food preparation and two quarts for sanitation: hand washing, teeth, dishes. The floor is a three-day supply; the recommendation for sheltering at home is two weeks. The calculator applies exactly that rule, then does the part the pamphlets skip: converting the total into containers you can actually picture buying, filling, and finding floor space for.
Run the two-week number for a family of four and you get 56 gallons, which surprises almost everyone the first time. As containers, that is eighteen 24-packs of bottled water, twelve 5-gallon stackable containers, or a bit more than one 55-gallon drum. Each form has real trade-offs - bottled water is rotation-free but bulky per gallon, drums are space-efficient but need a pump and a place you will never need to move them from - and the container comparison guide walks through them honestly.
Two adjustments the rule itself tells you to make: hot climates, children, nursing mothers, and illness all push needs up, so treat one gallon as a floor rather than an average; and if supplies ever run low, the official guidance is blunt - never ration drinking water. Drink what you need today and work on finding more, which is why the purification methods guide belongs next to this calculator: stored water is your first source, not your only one.
The stored-gallons field makes the tool an audit as well as a plan. Enter what is actually in the house right now and the result tells you how many days you currently cover and the exact shortfall in gallons. Coverage rounds down and container counts round up, deliberately: the arithmetic should always err on the side of you having margin. Pair the result with the food storage calculator - the two-week water figure and the two-week calorie figure are two halves of the same plan.