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Quick answer: Most healthy adults need roughly 9 to 13 cups (74 to 101 oz) of fluids a day. The National Academies reference intake is about 91 oz of total water a day for women and 125 oz for men, and roughly 80 percent of that usually comes from drinks, the rest from food. A simple personal rule is half an ounce of fluid per pound of body weight, plus about 12 oz for every 30 minutes of sweaty exercise. Enter your weight below for your own target.
Coffee, tea, milk and watery foods all count toward hydration; it does not have to be plain water.
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The most authoritative figures come from the US National Academies, which set an adequate intake of about 2.7 liters (91 oz) of total water a day for women and 3.7 liters (125 oz) for men. That total includes the water in food, which typically supplies about a fifth of it, so the part you drink works out to roughly 2.2 liters (74 oz, about 9 cups) for women and 3.0 liters (101 oz, about 13 cups) for men. These are averages for healthy people in a mild climate, not minimums: needs rise with body size, exercise and heat.
This calculator personalizes that with a widely used rule of thumb, half an ounce of fluid per pound of body weight, kept within a sensible 64 to 130 oz range, then adds fluid for exercise and hot weather. It also shows the National Academies figure for your sex, so you can see both.
| Body weight | Baseline (rest day) | With 30 min exercise |
|---|---|---|
| 100 to 128 lb (45 to 58 kg) | 64 oz (1.9 L, 8 cups) | 76 oz (2.2 L) |
| 140 lb (64 kg) | 70 oz (2.1 L, about 9 cups) | 82 oz (2.4 L) |
| 160 lb (73 kg) | 80 oz (2.4 L, 10 cups) | 92 oz (2.7 L) |
| 180 lb (82 kg) | 90 oz (2.7 L, about 11 cups) | 102 oz (3.0 L) |
| 200 lb (91 kg) | 100 oz (3.0 L, about 12 cups) | 112 oz (3.3 L) |
| 220 lb (100 kg) | 110 oz (3.3 L, about 14 cups) | 122 oz (3.6 L) |
| 250 lb (113 kg) | 125 oz (3.7 L, about 16 cups) | 137 oz (4.1 L) |
| 260 lb (118 kg) and over | 130 oz (3.8 L, cap) | 142 oz (4.2 L) |
Half an ounce per pound, bounded at 64 and 130 oz, plus about 12 oz per 30 minutes of sweaty exercise. 1 cup = 8 oz. Add more in hot weather, and spread it through the day.
The idea that coffee dehydrates you does not hold up at normal intakes. In a carefully controlled study, 50 regular coffee drinkers spent three days drinking four cups of coffee a day and three days drinking the same amount of water. Every measure of hydration, from total body water to blood and urine markers and 24-hour urine volume, came out the same. Milk, soup, fruit and vegetables count too. If you drink a lot of coffee, the caffeine half-life calculator shows how much is still in your system at bedtime.
In studies comparing urine color with lab measures of urine concentration, color tracked them closely enough to use as an everyday check. Pale straw-colored urine generally means you are drinking enough; dark yellow or amber means drink more. Some vitamins, especially B vitamins, and some foods change the color, so look at the pattern over the day rather than once.
Sweat rates vary so much between people that the American College of Sports Medicine recommends estimating your own: weigh yourself before and after a typical workout. Each pound lost is about 16 oz of fluid to replace, and the aim during exercise is to avoid losing more than 2 percent of your body weight. Drinks with electrolytes can help in long or very sweaty sessions.
Yes, and it is a real risk for some endurance athletes. Drinking far more than you sweat dilutes the sodium in your blood, a condition called hyponatremia, which can be dangerous. In a study of runners in the Boston Marathon, 13 percent of 488 finishers tested had low blood sodium, and 0.6 percent had critically low levels. The strongest risk factors were gaining weight during the race (a sign of drinking more than was lost), finishing in more than four hours, and being very underweight or overweight. For everyday life the lesson is simple: drink steadily through the day, let thirst guide you, and do not force large volumes in a short time.
It can help a little. In a 12-week trial of 48 adults aged 55 to 75 who were all on a lower-calorie diet, those who also drank 500 ml (about 17 oz) of water before each meal lost about 2 kg (4.4 lb) more than those who did not. The water seemed to make people eat slightly less at meals. Swapping sugary drinks for water also removes calories directly. Water does not burn fat on its own, but it is one of the easiest habits to add to a weight-loss plan. The fasting calculator and protein calculator cover two others.
Most healthy adults need about 74 to 101 oz (9 to 13 cups) of fluids a day, based on National Academies reference intakes of about 91 oz of total water for women and 125 oz for men, with roughly a fifth coming from food. A personal rule is half an ounce per pound of body weight, about 80 oz for a 160 lb adult, plus more for exercise and heat.
About 2.2 liters of fluids for women and 3.0 liters for men, on average, which together with the water in food makes up the National Academies totals of 2.7 and 3.7 liters. Larger or more active people need more. Using the half-ounce-per-pound rule, a 70 kg adult needs about 2.3 liters before exercise.
Yes. In a controlled study of 50 regular coffee drinkers, four cups of coffee a day hydrated just as well as the same amount of water across every measure tested. Tea, milk and other drinks count too. Sugary drinks hydrate but add calories, so water is the better default.
Yes. Drinking far more than you lose can dilute blood sodium (hyponatremia), which can be dangerous. It is most often seen in endurance events: 13 percent of Boston Marathon finishers tested in one study had low blood sodium, linked mainly to gaining weight during the race from overdrinking. Drink steadily and let thirst guide you rather than forcing large amounts.
Drink your normal daily target, and consider a glass of water before meals. In a 12-week trial of adults on a calorie-controlled diet, drinking 500 ml (about 17 oz) before each meal led to about 2 kg more weight loss than the diet alone. Replacing sugary drinks with water also cuts calories.
For your safety
Important: This target is for generally healthy adults. If you have kidney disease or heart failure, or take diuretics (“water pills”) or lithium, your fluid intake needs individual medical guidance; follow your clinician’s limits, not a calculator. Pregnancy and breastfeeding raise needs and deserve provider advice too.
This content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition.
