Daily water intake calculator

How the figure is built

A baseline of 35 ml per kilogram of body weight, plus 12 ml for every minute of exercise, scaled up by ten to twenty percent in warm or hot conditions. That gives total daily water. About a fifth of it arrives in food, more if you eat a lot of fruit, vegetables and soup, so the amount to actually drink is roughly four fifths of the total.

Coffee and tea count. The mild diuretic effect of caffeine at normal intakes does not offset the water in the cup, and that has been tested repeatedly. So do milk, juice and most soft drinks, though the sugar makes them a poor way to hydrate.

Where the eight glasses figure came from

A 1945 US Food and Nutrition Board recommendation of about 2.5 litres a day, immediately followed by a sentence noting most of it comes from prepared food. The sentence got dropped and the number stuck. It has been repeated for eighty years without ever having been the evidence-based rule people take it for.

A better indicator than any calculator

Urine colour. Pale straw means you are fine. Dark yellow means drink more. Completely clear all day means you are probably drinking more than you need, which is not harmful for most people but is not helping either. Thirst is also more reliable than it gets credit for in healthy adults under 65, and less reliable above that age, which is why older adults are advised to drink on a schedule rather than waiting for the signal.

When to ignore this and ask a doctor

Kidney disease, heart failure and some liver conditions come with fluid restrictions that override any general calculation, sometimes by a large margin. Certain medications change fluid balance too. If you have been given a fluid target by a clinician, that target wins and this calculator does not apply to you.

Before a procedure involving sedation, follow the clinic's fasting instructions rather than a hydration plan. They are specific for a reason.