Two ways to set a target
The popular US rule of thumb is half an ounce of water per pound of body weight per day. At 160 pounds that is 80 ounces, or 10 cups. It is not an official guideline, but it scales with body size in a way that "eight glasses a day" never did, and it lands close to the real numbers for most adults.
The official figures come from the National Academies of Sciences, Engineering, and Medicine, which set an adequate intake for total water of 3.7 liters (125 oz) a day for men and 2.7 liters (91 oz) for women. Total water includes everything you eat and drink. Since about 20% of it normally comes from food, the beverage share works out to roughly 13 cups for men and 9 cups for women, which is the number most often quoted. These are population averages set at a level that covers almost everyone, not a minimum each person must hit.
A worked example
Take a 160-pound man who exercises 30 minutes a day in a temperate climate. The baseline is 160 x 0.5 = 80 oz. Exercise adds about 12 oz for each 30 minutes of sweating, bringing it to 92 oz. Nothing is added for climate or life stage. Ninety-two ounces is 11.5 cups or 2.7 liters from drinks, comfortably inside the NASEM beverage target of about 101 oz for men once the food contribution is accounted for.
The adjustments matter more than the baseline. Hot or humid conditions, or working outdoors, add roughly 16 oz. Altitude above 8,000 feet increases fluid loss through breathing and adds about 8 oz. Pregnancy raises the NASEM total-water target from 2.7 to 3.0 liters, about 10 extra ounces; breastfeeding raises it to 3.8 liters, which is why nursing adds around 32 ounces here. Fever, vomiting, diarrhea and some medications raise needs further.
What counts, and when to ignore the formula
Every drink counts toward the total: water, coffee, tea, milk, juice, soda and sparkling water. The old claim that caffeine dehydrates you does not survive contact with the evidence; at normal doses the diuretic effect is small and the fluid in the cup more than compensates. Alcohol is the real exception, since it suppresses vasopressin and increases urine output. Food contributes too, especially fruit, vegetables, soup and yogurt, which is the 20% built into the NASEM numbers.
No formula beats your own body. Thirst is a reliable signal for healthy adults, and urine color is a good daily check: pale straw means you are fine, dark amber means drink more, and water that is completely colorless all day long may mean you are drinking more than you need. Drinking far beyond thirst is not harmless. Endurance athletes who overdrink during long events can develop exercise-associated hyponatremia, a dangerous dilution of blood sodium, and people with heart failure, advanced kidney disease or on a prescribed fluid restriction must follow their clinician's limit rather than any calculator.