Free water intake calculator
How much water, for your body and your day?
Scaled to your weight, your training and your climate — not the eight-glasses figure, which comes from a misread 1945 memo. You also get the part most calculators omit: how much of that total you actually need to drink, because roughly a fifth arrives in food.
General wellness guidance, not medical advice. If you are pregnant, breastfeeding, under 18, or managing a medical condition, talk to a doctor or a registered dietitian before acting on any of these numbers.
Where the numbers come from
| Component | Amount | Why |
|---|---|---|
| Baseline, under 55 | 35 ml per kg | Covers urine, breath, skin and stool losses in a temperate climate |
| Baseline, 55 and over | 30 ml per kg | Kidney concentrating capacity falls with age; thirst becomes less reliable too |
| Training | 500–750 ml per hour | Sweat rate varies enormously between people; this uses the midpoint |
| Hot or humid climate | +15% | Insensible losses rise sharply before you notice sweating |
| From food | about 20% of the total | So the figure to drink is smaller than the figure to consume |
The eight-glasses myth, briefly
The 1945 US Food and Nutrition Board recommendation said roughly 2.5 litres a day, and then immediately noted that most of that quantity is contained in prepared foods. The second half was dropped in the retelling, and eight glasses of pure water on top of everything else became folk wisdom. It is too much for a small sedentary person in a cool room and nowhere near enough for a large one training in Chennai in May.
Trust thirst over the number
For healthy people with intact thirst, the body regulates this better than any calculator can. Pale straw urine and no nagging thirst means you are fine. The estimate above is useful for the situations where thirst lags — long flights, hot work, older adults, and the first days of a new training block — not as a quota to force down.
NutriLog AI logs water against a daily target alongside food and weight, which is mostly useful as a nudge on the days you would otherwise forget, rather than as a rule to obey.
Common questions
How much water should I drink a day?
Around 35 ml per kilogram of body weight for most adults, dropping to about 30 ml/kg over 55 as the kidneys concentrate urine less efficiently — so roughly 2.5 litres for a 70 kg adult before any training or heat. Add about 500 to 750 ml per hour of real sweating. But treat that as a starting estimate, not a prescription: thirst is a better instrument than arithmetic for most healthy people.
Is it really eight glasses a day?
That figure has no serious basis. It appears to be a misreading of a 1945 US recommendation which said about 2.5 litres a day and then added, in the very next sentence, that most of it comes from food. The number stuck; the qualification did not. Your requirement scales with your body size, your training and your climate, not with a round number of glasses.
Does tea, coffee or food count?
Yes to all three. Roughly a fifth of daily water intake arrives in food — fruit, curd, dal, rice, anything cooked in water — which is why the "from drinks" figure above is smaller than the total. Tea and coffee count too: the diuretic effect of caffeine at normal intakes is far smaller than the water in the cup, so a cup of tea is a net gain.
Can you drink too much water?
Yes, and it is not merely wasteful. Drinking far beyond thirst over a short period can dilute blood sodium — hyponatraemia — which is dangerous and has killed endurance athletes and people attempting water-drinking challenges. Healthy kidneys clear roughly 0.8 to 1 litre an hour at most. There is no prize for exceeding your thirst.
How do I know if I am drinking enough?
Pale straw-coloured urine and no persistent thirst. That is the whole test, it is free, and it accounts for the day you trained in the heat as automatically as the day you sat indoors — which no calculator can. Dark urine, headache and fatigue point the other way.
Does drinking more water help you lose weight?
Only marginally and indirectly. A glass before a meal slightly reduces how much people eat in some trials, and drinks with calories replaced by water is a genuine saving. But water is not a fat-burner, and hydration will not rescue a calorie surplus. If weight loss is the goal, size the deficit and let water do its actual job.
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One number today, or a habit that sticks.
NutriLog AI tracks water alongside meals, macros and weight, so hydration is one tap rather than a separate app. Free, no ads, Android. Free on Google Play.