Water intake calculator

Where the number comes from

The baseline is 35 ml for every kilo you weigh. That scales with body size, which the eight glasses rule does not: a 95 kg man and a 52 kg woman do not need the same amount of anything, and telling them they do is why the rule gets ignored.

Exercise is added at roughly 450 ml per half hour, which is a middle estimate. Sweat rates vary more than almost any other measure in this field, from half a litre an hour to well over two in heat, so if you finish a session visibly salty or noticeably lighter, drink more than this says.

Heat adds half a litre. Cold takes a little off, though less than people expect, because you still lose water breathing dry winter air and you feel thirsty far less often.

How much water by body weight, in pounds

Most people ask this with a weight already in mind, so here is the same formula run at the weights people ask about most. These are for a day with no training in a temperate climate. Add about 15 fl oz for every half hour of exercise.

The popular shortcut of drinking half your body weight in ounces lands close to the total column, which already includes the water in food, and is then treated as if all of it has to come from a glass. A 200 lb person told to drink 100 oz is being asked for about 14 oz more than they need to drink.

In real heat: India, the Philippines, Malaysia, an Australian summer

Pick Hot on the calculator if you spend most of the day above about 30°C without air conditioning, or if it is humid enough that sweat does not dry on your skin. Humidity matters as much as the temperature, because sweat that drips off cools you less, so you keep sweating.

The half litre the calculator adds for heat is a floor rather than a ceiling. It covers a normal day indoors and out. A day working outside in Manila in May or Delhi in June can cost far more, and nothing but your own weight before and after will tell you how much.

Two things change in heat beyond the amount. Salt goes out with sweat, so on a long sweaty day a salted meal or a rehydration sachet does more than another plain litre. And the timing matters more: a glass every hour through the afternoon keeps up, where two litres at dinner mostly goes straight through.

Cups, glasses and ounces are not the same everywhere

Half the confusion about water targets is units. A cup and an ounce mean different amounts depending on where the advice was written, and a target copied from one country and measured in another can be off by a tenth.

The easiest way round all of it is to count bottles. Work out how many 500 ml or 750 ml bottles your number is, and refill that many times a day. It is the only unit you can see.

Drinking it, rather than reading about it

Most people who miss their intake do not misjudge the amount, they run out of moments. Three fixed points beat any reminder: a glass when you wake, one with each meal, and a bottle you refill at a set time. That is most of the way there before any conscious effort.

A bottle you like is worth more than an app. The measure that matters is how often it is within arm's reach.

Sources

• Dietary Reference Intakes for water, potassium, sodium, chloride, and sulfate, National Academies Press, 2005
• Water, drinks and hydration, NHS, 2023
• Exercise and fluid replacement, American College of Sports Medicine position stand, Medicine and Science in Sports and Exercise, 2007

How this was made

Written by Abdullah Javaid, founder of Calfit, drafted with AI assistance and edited by hand. The total is 35 ml per kilogram of body weight, plus about 450 ml for each half hour of exercise, plus half a litre in hot weather, with 150 ml taken off in the cold. A fifth of the total is assumed to come from food, and a glass is counted as 240 ml.

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