How much water you need

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Official guides (CDC, Ready.gov) say to store at least 1 gallon (3.8 L) per person per day for 3 days, and preferably for 2 weeks. That gallon covers drinking (about ¾ of a gallon for a normally active person) and a minimum for cooking and hygiene. Pets count too: a dog of about 15 lb (7 kg) drinks about 1.5 pints (0.7 L) a day, more in heat (detailed calculation in chapter 2, section 2.2).

How much water you need
Per person (plus water for pets based on their weight)	Per day	3 days (minimum)	7 days	14 days (recommended)
Mild weather (1 gal per person per day)	1 gal (3.8 L) per person	3 gal (11.4 L) per person	7 gal (26.5 L)	14 gal (53 L) per person
Extreme heat (need up to double)	2 gal (7.6 L)	6 gal (23 L)	14 gal (53 L) per person	28 gal (106 L)

These people need more water: pregnant women, nursing mothers, children, the sick, older adults, and anyone doing physical work in the heat. The Army manual (FM 21-76) says an adult loses and needs 2 to 3 liters a day at rest at 68 °F (20 °C), and that in extreme arid climates the need can reach 14–30 liters a day with activity.

How to stretch your water without rationing

Reduce physical activity and stay in the shade or in the coolest room.
Work at night or early in the morning if you have to exert yourself.
Don't replace water with soda, coffee, energy drinks, or alcohol: they dehydrate.
Eat little if you have very little water: digestion uses up water (especially protein).
Drink in sips and regularly. Almost nobody can comfortably drink more than 1 liter at once.
Use non-drinkable water (pool, untreated rain) for flushing the toilet and cleaning, never for drinking.
If you can't treat questionable water, put off drinking it as long as you can without becoming dehydrated. When there is no alternative left, drink: dehydration kills faster than most germs.