Water intake calculator: how much water to drink a day

How it works

Start with the European Food Safety Authority’s adequate intake of total water: 2.0 liters a day for women (about 68 fl oz) and 2.5 for men (85 fl oz); 2.3 when pregnant (78 fl oz) and 2.7 when breastfeeding (91 fl oz). It counts all drinks plus the water in food, in mild weather with moderate activity, and applies from age 14.
Add 0.5 liters (about 17 fl oz) for each hour of exercise and 0.5 liters for a hot day. These two are cautious estimates: athletes in a large study lost about 0.8 to 1.5 liters of sweat an hour on average, depending on the sport.
About 20% of the water we take in comes from food, according to the US National Academies. So the part to drink is 80% of the reference, plus all the extra for exercise and heat.

Worked example

A woman who exercises 1 hour a day: 2.0 L + 0.5 L = 2.5 L of water in all, about 85 fl oz. Food brings about 20% of the reference, 0.4 L, so she needs to drink about 2.1 L: 71 fl oz, roughly nine 8-ounce glasses. A man with 2 hours of exercise on a hot day: 2.5 + 1.0 + 0.5 = 4.0 L in all (135 fl oz), 3.5 L to drink (118 fl oz).

Sources

Text checked on September 30, 2026

Good to know

Most healthy people meet their needs by drinking when they’re thirsty, as the US National Academies put it. Treat this number as a reference, not a quota.
US references are higher: about 2.7 liters (91 fl oz) of total water a day for women and 3.7 liters (125 fl oz) for men, based on what healthy Americans typically take in. The EFSA values used here are for mild weather and moderate activity.
Exercise and heat change your needs the most. The 0.5 liters an hour here is cautious: to know your own sweat rate, weigh yourself before and after a workout, as the American College of Sports Medicine suggests. Each kilogram lost is about a liter of sweat; add back what you drank.
You can drink too much. EFSA notes that adult kidneys can pass only about a liter of urine an hour, and much more water than that can dangerously dilute the blood.
If you have a heart, kidney or liver condition, go by your doctor’s advice on fluids, not this page: with heart failure, for example, your provider may tell you how much to drink.

Frequently asked questions

How much water should I drink a day?

It depends on your sex, how active you are and the weather. As a reference, EFSA puts total water at 2.0 liters a day for women and 2.5 for men, water in food included; about 80% of it comes from drinks. In the UK, the NHS suggests 6 to 8 cups or glasses of fluid a day.

Do coffee and tea count?

Yes. The National Academies count caffeinated drinks toward your water, and the NHS says water, lower-fat milk and lower-sugar drinks, including tea and coffee, all count.

Is 8 glasses a day a rule?

No. The National Academies say they don’t offer a rule of thumb for how many glasses to drink, because all drinks and food count. For reference, eight 8-ounce glasses are 64 fl oz, about 1.9 liters.

How much more should I drink when I exercise?

It depends on how much you sweat. Athletes in a large study lost about 0.8 to 1.5 liters an hour on average, depending on the sport. The American College of Sports Medicine suggests weighing yourself before and after to find your own rate, and not losing more than 2% of your body weight in water during exercise.

Can you drink too much water?

Yes, although it’s rare. EFSA explains that drinking more than the kidneys can clear, about a liter of urine an hour in adults, dilutes the blood and can be dangerous.

Water intake calculator

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