Gallons in plain numbers, then the containers to hold them. The standard advice — one gallon per person per day — is a fine slogan and a poor shopping list. This calculator adjusts that gallon for hot weather and hard work, adds the pets everyone forgets, and then translates the total into stackable containers, barrels, and cases, with the weight included so you can decide where it all can actually live.
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Where does the one-gallon rule come from?
Emergency guidance has used one gallon per person per day for decades: roughly half for drinking, half for cooking and the barest hygiene. It is a floor for mild weather and a quiet house. Heat, physical work, illness, and nursing or pregnant household members all push it up — which is why this calculator adjusts instead of just repeating the slogan. Two gallons per person per day is the comfortable version, and if you have the space, nobody regrets storing toward it.
What about the water already in the house?
A water heater holds 30–50 gallons of drinkable water and the pipes hold a few more — a real bonus, and the Water hub explains how to get at it safely. Count it as a cushion, not as your stored number: heaters fail, valves stick, and in some emergencies the supply is contaminated before you shut it off. Stored water you control beats water you're counting on. Water is week one of the first-month guide because everything else in a hard week waits on it.