The Institute of Medicine says adult men need about 13 cups (3 liters) per day of fluid; adult women need about 9 cups (2.2 liters) of fluid. (You get about an additional 21/2 cups of fluid from foods.)
“But one size doesn’t fit all,” says Leslie Bonci, R.D., C.S.S.D., director of sports nutrition at the Center for Sports Medicine at the University of Pittsburgh Medical Center and dietitian for the Pittsburgh Steelers. Your size and activity level affect your fluid requirements. Simply put, the larger and more active you are, the more you’ll need.
“The easiest thing that anybody could do on a daily basis is monitor their urine color,” says Douglas Casa, Ph.D., A.T.C., who studies hydration at the University of Connecticut. “Lighter urine color—like lemonade—means you’re generally well-hydrated. If it’s darker, like apple juice, you are most likely dehydrated.”
Older adults’ fluid needs don’t change, but they’re more likely to become dehydrated because their sense of thirst declines. Pregnant women and nursing mothers need slightly more water. Some medications, such as antihistamines and certain antidepressants, increase your fluid needs too.