Water intake calculator: how much to drink a day
An adult woman usually needs about 2.2 litres of drinks a day and a man about 3 litres. That's the adequate intake from the NASEM (US National Academies) report, 2005: 2.7 and 3.7 litres of total water, with about 19% coming from food. In hot weather the calculator adds about 0.8 litres, and with high activity the number grows with your calorie burn. Enter your details for litres and glasses.
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Where the number comes from
The formula, a worked example and where it stops working
How much water should I drink a day: the short answer
Most adult women do well on about 2.2 litres of drinks a day and men on about 3 litres. That is the adequate intake from the NASEM water intakes report, 2005: 2.7 and 3.7 litres of total water, about 19% of which comes from food. With a lot of activity the need grows with your calorie burn, and in hot weather by at least a litre.
In 250 ml glasses that is roughly 9 for a woman and 12 for a man. The calculator above does the same maths for your own weight, height, age and activity.
How we calculate
It starts from the adequate intake for total water in the NASEM water intakes report, 2005: 2.7 litres for women and 3.7 for men. This is not a survival minimum. It is roughly what healthy, well-hydrated adults actually take in from drinks and food.
Then it compares that with a second yardstick from the same report: 1 ml of water for every kilocalorie you burn. Your daily burn is worked out the same way as on the other calculators here — resting metabolism from Mifflin MD, St Jeor ST et al., American Journal of Clinical Nutrition, 1990, times an activity factor. The larger of the two wins. That is why weight and activity only move the result for big, very active people: everyone else burns less than 2,700 or 3,700 kcal, so the standard intake applies.
If you tick hot weather, a litre is added to total water. The NASEM report says needs range from 3 to 6 litres a day in temperate climates and from 4 to 12 litres in hot ones, depending on activity. One litre is the smallest shift, not a ceiling: working in the sun takes far more.
Finally, 81% of total water is shown as drinks — that is the share drinks made up in the same data. The rest comes from food: vegetables, fruit, porridge, soup.
Example: a 30-year-old woman, 65 kg, 165 cm, lightly active. The calculator puts her burn at about 1,884 kcal, which is under 2,700, so the standard intake applies: 2.7 litres of total water, 2.2 litres of it from drinks. That is about 9 glasses.
The CalCash bot sets your water target with the same formula: during setup it works out the same drinks figure as this calculator, without the hot-weather extra.
Water intake for women by weight and activity
Same engine as the calculator. The number barely moves until activity gets very high, while hot weather adds almost a litre of drinks straight away.
| Weight | Sedentary | Moderate | Very active | Sedentary, hot |
|---|---|---|---|---|
| 50 kg | 2.2 L | 2.2 L | 2.2 L | 3.0 L |
| 60 kg | 2.2 L | 2.2 L | 2.2 L | 3.0 L |
| 70 kg | 2.2 L | 2.2 L | 2.2 L | 3.0 L |
| 80 kg | 2.2 L | 2.2 L | 2.3 L | 3.0 L |
| 90 kg | 2.2 L | 2.2 L | 2.5 L | 3.0 L |
| 100 kg | 2.2 L | 2.2 L | 2.6 L | 3.0 L |
| 110 kg | 2.2 L | 2.3 L | 2.8 L | 3.0 L |
Woman, 30, 165 cm. Drinks only, water from food not included. Based on NASEM 2005, the same maths as in the calculator.
Water intake for men by weight and activity
The adequate intake for men is higher — 3.7 litres of total water, about 3 litres of it from drinks. Calorie burn overtakes it only for very large, very active men.
| Weight | Sedentary | Moderate | Very active | Sedentary, hot |
|---|---|---|---|---|
| 50 kg | 3.0 L | 3.0 L | 3.0 L | 3.8 L |
| 60 kg | 3.0 L | 3.0 L | 3.0 L | 3.8 L |
| 70 kg | 3.0 L | 3.0 L | 3.0 L | 3.8 L |
| 80 kg | 3.0 L | 3.0 L | 3.0 L | 3.8 L |
| 90 kg | 3.0 L | 3.0 L | 3.0 L | 3.8 L |
| 100 kg | 3.0 L | 3.0 L | 3.0 L | 3.8 L |
| 110 kg | 3.0 L | 3.0 L | 3.2 L | 3.8 L |
Man, 30, 178 cm. Drinks only, water from food not included. Based on NASEM 2005, the same maths as in the calculator.
Why not «30 ml per kilogram»
The «30-35 ml per kilo» rule shows up in almost every article about water. The NASEM report we rely on has no such rule for adults: it ties water to energy burn, climate and how much people really drink. So the calculator does not use it.
For someone of 60-80 kg the per-kilo rule and the NASEM intake land close to each other. They split at the edges: per kilo gives a very light person too little and a very heavy person a lot, although a heavier body does not always lose more water.
What counts as water
NASEM counts all water: plain water, every drink and the water in food. Tea, coffee, milk and soup all count as drinks. You do not have to get the whole amount from plain water.
A simpler guide than any number is thirst and the colour of your urine. As a common rule of thumb, pale straw means you are drinking enough. Dark urine and thirst by the evening are a sign to drink more, especially in the heat and after training.
When to drink more, and when to be careful
You need more in hot weather and during exercise — NASEM says so directly. In the heat or working in the sun, needs can be several times what the calculator shows: drink to thirst and do not wait until it gets strong.
Be careful with kidney or heart disease, with swelling, or if a doctor has already limited your fluids. The calculator is not for you then — your doctor sets the number.
And do not drink it all at once. The same report says the kidneys clear no more than 0.7-1 litre of water an hour, and water poisoning has happened to people who drank far more than that in a short time. Spread it over the day.
A target is easy to work out and just as easy to forget by lunchtime. In the CalCash bot you can log water right in the chat next to your meals, and it counts the meals from a photo, voice or text.
Frequently asked questions
How much water should I drink per kg of body weight?
NASEM has no per-kilogram rule for adults. Its intake is about 2.7 litres of total water for women and 3.7 for men, of which 2.2 and 3 litres come from drinks. The calculator brings weight in through your calorie burn, so big, active people get a higher number.
How much water should I drink a day to lose weight?
The same as usual. Water does not burn fat on its own; you lose weight through a calorie deficit. The calorie deficit calculator works that out, and the weight loss calculator shows the date you reach your goal.
How much water should a man drink a day?
By NASEM intakes an adult man needs about 3.7 litres of total water a day, about 3 litres of it from drinks — roughly 12 glasses. More with high activity and in hot weather.
How much water is too much in a day?
NASEM sets no upper limit for healthy people, but the kidneys clear only about 0.7-1 litre an hour. The danger is drinking a lot in a short time, not over a whole day. With kidney or heart disease your fluid target comes from a doctor.
Do tea, coffee and soup count towards water intake?
Yes. NASEM counts all water from drinks and food. Tea, coffee, milk and soup all count as drinks, and about a fifth of your total water comes from food anyway.
Reminders
Water and weight