Water Intake Calculator

There are two ways to answer this and they disagree politely. The official figures are population adequate intakes: the US reference values put total water at 3.7 L a day for men and 2.7 L for women, and the European values at 2.5 L and 2.0 L. Those are fixed numbers that take no account of how big you are.

The clinical shorthand scales instead: 30 to 35 ml per kilogram of body weight a day. This calculator uses the midpoint of that and prints the range either side of it, because the range is the honest answer.

US reference (total water)

3.7 L men · 2.7 L womenAll drinks and food combined

European reference (total water)

2.5 L men · 2.0 L womenAll drinks and food combined

Clinical rule of thumb

30 – 35 ml per kg per dayScales with body size; the basis of this tool

Exercise

≈ 500 ml per hourMore in heat, and highly individual

The official figures are for TOTAL water, which includes what you get from food — roughly a fifth of intake for most people. A target for drinks alone is lower than the headline number.

Nearly everything you drink counts. Tea, coffee, milk, juice and soft drinks are mostly water and contribute accordingly; the old claim that caffeine dehydrates you does not hold at ordinary intakes, because the diuretic effect is smaller than the volume of the drink.

• Food supplies roughly 20 per cent of total water intake — more if you eat a lot of fruit, vegetables and soup.
• Alcohol is the genuine exception: it suppresses the hormone that conserves water, so it costs you more than it supplies.
• Thirst is a reliable signal for healthy adults under 65. It becomes less reliable with age, which is why older adults are advised to drink to a schedule rather than to thirst.
• Pale straw-coloured urine is the simplest day-to-day check. Consistently dark suggests you are behind; consistently clear suggests you are ahead of what you need.

Sweat rates vary enormously — from well under half a litre an hour to over two litres in heat. The 500 ml allowance here is an average and nothing more; if you train seriously, weighing yourself before and after a session tells you your own rate in a way no equation can.

• Heat, humidity and altitude all raise losses, sometimes sharply.
• Fever, vomiting and diarrhoea raise them too, and are the situations where electrolytes matter alongside water.
• Diuretics, SGLT2 inhibitors and some other medicines change fluid balance directly.
• Pregnancy and breastfeeding raise requirements above these figures.
• Heart failure, kidney disease, liver disease and some hormonal conditions can require a fluid restriction — a limit, not a target.

And more is not simply better. Drinking far more plain water than you lose, particularly during prolonged exercise, can dilute blood sodium — exercise-associated hyponatraemia — which is dangerous. The target is a target, not a score.

Millilitres a day = (body weight in kg × 32.5) + (hours of exercise × 500), with the range either side of it calculated at 30 and 35 ml per kg.

An 84 kg adult training for an hour: (84 × 32.5) + 500 = 3,230 ml, about 3.2 L, inside a published range of 3.0 to 3.4 L. Imperial entries are converted to kilograms before the calculation, not after.