How much, roughly
The familiar eight glasses a day is a rule of thumb with no real research behind it. The National Academies put adequate total water at about 3.7 liters a day for men and 2.7 liters for women, and that figure includes food, which supplies around a fifth of it. Milk, tea and coffee all count. Caffeine is a mild diuretic, but the water in a cup of coffee more than covers what it costs you. Needs rise with heat, altitude, exercise, pregnancy, breastfeeding and illness, so a fixed number is less useful than the daily signals.
Reading the signals
Besides urine color, expect to urinate every two to four hours during the day. The early signs of running low are a headache, fatigue, poor concentration, a dry mouth and dizziness when you stand up. One caveat applies to the color test: B vitamins in a multivitamin turn urine bright yellow for a few hours regardless of how much you have drunk.
Exercise and heat
For a workout under an hour, water is all you need. Sports drinks earn their sugar only in hard efforts lasting longer than about an hour, or in heavy sweating in the heat, when the sodium they carry matters. To learn your own sweat rate, weigh yourself before and after a long session: each pound lost is roughly sixteen ounces of fluid to replace over the next few hours.
When working or exercising in serious heat, the standard guidance is about eight ounces every fifteen to twenty minutes, started before you feel thirsty. Alternate alcoholic drinks with glasses of water, since alcohol suppresses the hormone that tells the kidneys to hold water.
Too much is also possible
Drinking far more than you lose dilutes the sodium in the blood, a condition called hyponatremia that turns up mostly in endurance events. The symptoms are nausea, headache, confusion and swelling, and severe cases are emergencies. Do not force down more than about a quart and a half in an hour.
When you are sick
Vomiting and diarrhea drain salts as well as water. Take small, frequent sips of an oral rehydration solution from the pharmacy, which the body absorbs better than plain water or soda. Seek care if you cannot keep fluids down for a day, have not urinated in eight hours, or feel confused or faint.
Tip Wash a reusable bottle daily with hot soapy water, including the lid and straw, because the film that forms inside is bacteria.
