Water intake calculator: how much water should I drink?

How much water you need, in ounces and glasses

This water intake calculator answers a common question: how much water should I drink a day? It gives two figures. The first is your recommended total intake, which counts all the water you take in, from drinks and from food such as fruit, vegetables, soup and dairy. The second is the amount to drink, shown in fluid ounces, liters, glasses and bottles so it is easy to follow.

The starting point is the adequate intake set by the European Food Safety Authority: 2.0 liters a day for women and 2.5 liters for men, in total water, from age 14. The calculator then adds pregnancy, breastfeeding, exercise and heat. The result is an estimate: real needs vary from one person to the next.

How to use it

1. Keep "US (lb, oz)" or switch to "Metric (kg, L, cl)", then pick "Female" or "Male".
2. Fill in "Age (years)" and "Weight (lb)".
3. For a woman, choose under "Pregnancy or breastfeeding": "Neither", "Pregnant" or "Breastfeeding".
4. Enter "Exercise (minutes per day)" and the "Exercise intensity": "Light", "Moderate" (the default) or "Hard".
5. Pick the "Climate": "Temperate", "Hot (77 to 86 °F)" or "Very hot (above 86 °F)".
6. If your glasses are not 8 oz, change "Glass size (fl oz)".

The result appears straight away. In "Today's tracker", tap a glass each time you drink, or press "+ 1 glass". The count starts again from zero at midnight.

How the estimate works

The calculator starts from the base (2.0 L or 2.5 L) and adds 0.3 L during pregnancy or 0.7 L while breastfeeding. About 20% of that comes from food; the rest is to drink. Exercise is added on top, all of it to drink: 0.4 L per hour of light effort, 0.6 L per hour of moderate effort, 0.8 L per hour of hard effort. Heat adds 0.5 L in hot weather and 1.0 L in very hot weather. These last two are rough averages, since sweating varies a lot. One US fluid ounce is 29.57 mL.

For the man: 2.5 L base plus 60 min × 0.6 L per hour = 0.6 L, so 3.1 L in total. Take away the 0.5 L that comes from food and 2.6 L, about 88 oz, is left to drink. That is 11 glasses of 8 oz, or 5.2 bottles of 16.9 oz. For the breastfeeding woman: 2.0 + 0.7 = 2.7 L, of which 0.54 L comes from food, leaving 2.16 L to drink.

The calculator also shows the common rule of 30 to 35 mL per kg (about half an ounce per pound) as a point of comparison: 61 to 71 oz for 132 lb, 81 to 94 oz for 176 lb.

Drink enough, but don't overdo it

Thirst is still a good everyday guide. It gets weaker with age, so older adults do well to drink regularly in small amounts, especially during a heat wave. Dark and infrequent urine, a dry mouth, tiredness, headaches or dizziness can point to dehydration.

On the other hand, there is no need to force yourself. Too much water does happen and can lower blood sodium to dangerous levels, especially during a marathon or a long hike where you drink a lot without eating. If you have heart or kidney failure, take diuretics or have been told to limit fluids, follow your doctor's advice, not this calculator.

Frequently asked questions

Should I really drink eight glasses of water a day?

The "8 × 8" rule (eight 8 oz glasses, 64 oz) is close to the result for a woman who does not exercise (54 oz to drink) and below what an active man needs. Coffee, tea, milk and soup also count toward the amount to drink.

Why is the total intake higher than the amount to drink?

Because food contains water. A diet with plenty of fruit and vegetables supplies about 20% of the total, and the calculator takes that share away.

What happens to the tracker at midnight?

The number of checked glasses goes back to zero. Only that count is kept on your device; your settings are kept only if you check "Also keep my settings on this device".

Is this calculator suitable for children?

No. It is meant for teenagers from 14 and adults. Under 14, needs depend on age and growth, so ask a doctor.