A person can live a month without food but only a few days without water.
Water: one gallon per person, per day.
According to FEMA (Federal Emergency Management Agency), in case of an emergency, you should store at least one gallon of water per person per day and have at least a three-day supply. However, individual water needs vary per person, depending on age, physical condition, activity, and climate. Children, nursing mothers, and ill people need more water and very hot temperatures can double the amount of water needed.
If you have pets, allow a minimum of a gallon per day for each dog or cat.
Contaminated water can cause such diseases as dysentery, typhoid, and hepatitis.
If unsure about the water, bringing it to a rolling boil for 1 minute will kill most organisms.