Water Intake Calculator

Where the recommendations come from

The European Food Safety Authority sets adequate intake at 2.5 litres a day for men and 2 litres for women, from all sources including food.

The US Institute of Medicine sets 3.7 litres for men and 2.7 for women, again from all sources. The difference between the two reflects different populations and methods rather than a disagreement about physiology.

Food supplies roughly 20% to 30% of the total, more for anyone eating substantial fruit and vegetables. Which means the amount that needs drinking is meaningfully less than the headline figure.

The eight glasses story

The advice to drink eight glasses of water a day has no clear scientific basis and appears to derive from a 1945 US Food and Nutrition Board recommendation.

That recommendation specified around 2.5 litres and included a sentence noting that most of this quantity is contained in prepared foods. The sentence was dropped in subsequent retellings and the figure became eight glasses of water in addition to everything else.

It is not harmful advice for most people and it is not evidence-based, and the difference matters because it gets repeated with a confidence the evidence does not support.

What raises the requirement

Exercise is the largest factor. Sweat rates range from around 0.5 to over 2 litres an hour depending on intensity, heat and individual physiology.

Heat and humidity raise losses independently of exercise. Altitude increases respiratory water loss. Fever raises requirements, as do vomiting and diarrhoea, which is why rehydration matters in gastrointestinal illness.

Pregnancy and breastfeeding both increase requirements, breastfeeding substantially, since milk production consumes water directly.

What counts

All fluids count, including tea and coffee. The belief that caffeinated drinks are net dehydrating is not supported: the diuretic effect at normal intakes is modest and outweighed by the fluid volume consumed.

Milk, juice, soup and soft drinks all contribute. Alcohol genuinely does not, since it suppresses antidiuretic hormone and increases urine output beyond the volume drunk.

Food contributes substantially. Fruit, vegetables, yoghurt and cooked grains all carry significant water, and someone eating a varied diet needs less drinking water than the general figures imply.

Too much is also possible

Drinking far beyond requirements can dilute blood sodium, producing hyponatraemia, which in severe cases causes seizures and can be fatal.

It is rare and it happens, most often in endurance events where participants drink heavily during prolonged exercise, and occasionally in water-drinking challenges.

The kidneys can excrete roughly 0.8 to 1 litre an hour at maximum, so intake sustained above that outpaces excretion. For almost everyone, drinking to thirst prevents both dehydration and overhydration, and thirst is a more reliable guide than any calculated target.

There is one group for whom drinking to thirst is genuinely not sufficient, and it is worth naming. The thirst response blunts with age, which is why dehydration is both more common and more consequential in older adults, and it can present as confusion or a fall rather than as thirst. In hot weather, or during illness, prompting fluid intake on a schedule rather than waiting for thirst is a reasonable precaution for anyone in that group, and it is one of the few situations where a target figure is more useful than the body's own signal.