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The standard recommendation of 8 glasses (64 oz) per day is a general guideline not a medical prescription. A more accurate formula is 0.5 to 1 oz of water per pound of body weight daily. A 160-pound adult needs 80 to 160 oz — 10 to 20 cups — per day. Active people and those in hot climates need significantly more.
Daily water needs vary significantly by body weight activity level climate and health status. The National Academies of Sciences recommends 3.7 liters (125 oz) per day for men and 2.7 liters (91 oz) per day for women from all beverages and food combined. About 20% of daily water intake comes from food.