Water Intake Calculator
Enter your weight, how long you exercise and what the weather is like. We estimate how much water you should drink in a day.
This calculator gives general estimates for information only. It is not medical advice and does not replace a consultation with a doctor or other qualified health professional.
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The formula
Water (ml) = 35 x body weight (kg) + 350 per 30 minutes of exercise + 500 in hot weather.
Where the number comes from
There is no single rule, but a common starting point is about 35 millilitres per kilogram of body weight. We add 350 ml for every half hour of exercise, because you sweat water out, and a further 500 ml if it is hot or humid.
An example
A 70 kg person who exercises for 30 minutes on a mild day: 70 x 35 = 2,450 ml, plus 350 ml, so about 2.8 litres, or around 11 glasses of 250 ml.
It includes food
About 20 percent of your water comes from food, and tea, coffee and milk count too. You do not need to drink the whole amount as plain water.
Signs you are drinking enough
Pale yellow urine and rarely feeling thirsty are good signs. Dark urine, headaches and tiredness can point to too little.
Needs are higher with illness, fever, pregnancy and breastfeeding. People with kidney or heart conditions are often told to limit fluids, so follow your doctor's advice.
Examples to check
Type these into the calculator above to see how it behaves. Every result below was produced by the same calculator code that runs on this page.
| What you enter | What you get |
|---|---|
| Weight (kg): 70; Exercise per day (minutes): 30; Mild | Suggested daily water: 2.8 litres (95 fl oz) |
| Weight (kg): 55; Mild | Suggested daily water: 1.9 litres (65 fl oz) |
| Weight (kg): 85; Exercise per day (minutes): 60; Hot or humid | Suggested daily water: 4.2 litres (141 fl oz) |
Water Intake Calculator: frequently asked questions
Is eight glasses a day a real rule?
It is a rough guide that stuck. Your needs depend on your size, activity, climate and diet.
Can you drink too much water?
Yes, though it is rare. Drinking several litres in a short time can dilute the salt in your blood. Spread your drinking through the day.
Can I rely on this result as medical advice?
No. It is a general estimate for information only. It cannot replace a check-up, and your doctor can interpret the numbers for your own situation.
Is what I enter in this calculator saved or sent anywhere?
No. The calculation runs in your browser, so what you enter is not sent to our servers. We only count, anonymously, whether a tool ran successfully.
Is this calculator free to use?
Yes. It is free, needs no sign-up and has no usage limit.
Last reviewed: October 3, 2026