What this estimate does not cover
How to use this calculator
- Choose your sex, life stage and activity level.
- Enter your age.
- Read the figure for drinks — not the total, which also includes water in food.
- Copy the link to keep this result.
The values shown are adequate intakes, not requirements. The Institute of Medicine uses the word deliberately: they are the intakes judged sufficient for almost everyone in a healthy group, which is a lower bar than what an individual might need.
The formula
The published values are 3.7 L/day for adult men, 2.7 L for adult women, 3.0 L in pregnancy and 3.8 L while breastfeeding. Men over 60 drop to 3.0 L and women over 60 to 2.4 L. The 0.81 is the Institute's own estimate of the share coming from beverages, with the remaining 19% assumed to arrive in food.
Two rules this page refuses to use
"Drink 30–35 ml per kg of body weight." This is the most common rule on water calculators and it has no primary source. It does not appear in the IOM report, and the physiologists who wrote it note that the per-kilogram approach produces figures that are too high for people who are sedentary and too low for people who are very active or who live in a hot climate. Since it has no defensible derivation, weight is not an input on this page at all.
"Add 500 ml for every hour of exercise." Measured sweat rates span 0.5 to 4.0 litres per hour depending on conditions, so a single hourly figure cannot be right for everyone — it is far too little for a runner in summer and absurd for a manual worker. Activity is therefore a three-way choice worth a fixed difference, taken from the observed spread in the IOM's own data.
Neither of these is a criticism of you or of anyone who quotes them. They are reasonable pieces of folk advice. They just are not sourced, and this project does not publish unsourced numbers.
The limitation that matters most here
These figures assume a temperate climate. The IOM states that adequate intakes apply to people in temperate conditions, and that needs in hot weather or heavy work can reach 6–8 litres a day.
That is a very large gap, and it is the single most important thing to know on this page. If you are somewhere genuinely hot, the figure below is a floor and not a target. Thirst is a reasonably good guide, and pale yellow urine is a reasonable check that you are keeping up.
Source
Institute of Medicine (2005). Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. National Academies Press. The Institute advises that these become the Dietary Reference Intakes for the United States and Canada.
Population covered: adults 19 and older, plus separate values for pregnancy and lactation. There are no adequate intakes for children on this page, which is why the age field starts at 19. The AIs are described as applying to healthy, sedentary people in temperate climates who have access to adequate food and beverages.
Frequently asked questions
Does this count tea, coffee and juice?
Yes, as beverages. The 81% share covers all drinks, and the IOM counts plain water as roughly 60% of that with about 20% each from milk and other beverages.
Does the food really count for 19%?
Yes. The IOM estimates roughly 20% of water intake comes from solid food, and applied a slightly conservative 19% share when deriving its beverage volumes.
What if I weigh 95 kg?
It makes no difference here, and that is deliberate. The per-kilogram rule has no source, so your weight is not asked for.