How much water do you need? The honest answer is a range

The eight-glasses rule has no study behind it

In 2002 a physiologist went looking for the evidence behind "drink at least eight glasses of water a day" and published what he found, which was nothing. No scientific studies in support of it. Survey data on healthy adults suggested that much was not needed.

His best guess at where it came from is more interesting than the rule. A 1945 US Food and Nutrition Board recommendation suggested roughly one millilitre of water per calorie of food, which lands around 64 to 80 ounces a day. The very next sentence said most of that quantity is contained in prepared foods. That sentence appears to have been dropped, and the number survived without it.

His conclusion was carefully scoped, and it is worth keeping the scope: healthy adults, temperate climate, largely sedentary. Not athletes in heat, not people who are ill.

The millilitres-per-kilogram formula is not from anywhere either

You will see hydration targets built from your body weight, usually somewhere between 30 and 45 millilitres per kilogram. This is worth being blunt about: no reference body publishes that rule for healthy adults, and the figures do not trace to a primary source. They circulate through supplement marketing and app copy, in the same way the 50-calorie fasting rule does.

Per-kilogram rules do exist, which is probably how the confusion started, but every one of them belongs to a different population or a different question.

This app used to size its own hydration target that way, and credited it to bodies that never said it. That has been corrected, and the number you see now is built from the reference intakes below.

What the reference bodies do publish

Flat daily figures by sex, not formulas. The US National Academies set adequate intakes of 3.7 litres of total water a day for men and 2.7 litres for women aged 19 to 30. Of that, they report drinks supplying about 3.0 litres for men and 2.2 for women, with the remainder coming from food.

EFSA sets 2.5 litres a day for men and 2.0 for women, and those figures include water from food. EFSA also attaches a condition that usually gets dropped when the number is quoted: moderate environmental temperature and moderate physical activity.

One thing to understand about all of these: an adequate intake is not a requirement. The National Academies say so directly, noting that intake below the figure does not by itself confer risk, because a wide range of intakes is compatible with normal hydration.

Why one number for everybody was never going to work

The largest study of this measured water turnover in 5,604 people aged eight days to 96 years across 23 countries, using isotope-labelled water rather than asking people what they drank. Age, body size and composition, physical activity, athletic status, pregnancy, and environment, including latitude, altitude, temperature and humidity, were all significantly associated with how much water moved through a person.

The averages alone make the point. Men aged 20 to 35 turned over about 4.2 litres a day, falling to around 2.5 litres in men in their nineties. Women aged 20 to 40 averaged about 3.3 litres, declining to roughly 2.5 by age 90. The study's senior author said plainly that a one-size-fits-all policy for water intake is not supported by the data, and that the two-litre suggestion is probably too high for most people.

Water turnover is not the same as water you have to drink, and this is where the number usually gets mangled. Turnover includes water from food and water your body produces itself. The same team put actual drinking needs nearer 1.5 to 1.8 litres a day for a young adult man and 1.3 to 1.4 for a woman in her twenties.

Coffee and tea count

This is the most persistent piece of misinformation in the category. Caffeinated drinks contribute to your daily water intake.

The National Academies address it directly: while caffeine and alcohol have shown diuretic effects in some studies, the available information indicates this may be transient, and such beverages contribute to total water intake.

A controlled trial makes the same point more concretely. Fifty habitually caffeinated men drank either four 200 ml cups of coffee a day or the same volume of water, in a counterbalanced crossover. Across a wide range of blood and urine hydration markers there were no significant differences between the two. The authors concluded that coffee, drunk in moderation by people used to caffeine, hydrates about as well as water does.

You can drink too much, and it is not a theoretical risk

Drinking fluid in excess of what you lose through sweat and urine is the primary cause of hyponatraemia, meaning blood sodium below 135 mmol per litre. It is also called water intoxication.

Below roughly 130 mmol per litre the symptoms run from bloating, puffiness, weight gain, nausea and headache through to confusion, seizures, respiratory distress and, untreated, death. Heavy sweat sodium losses and low-sodium sports drinks make it worse, and so does drinking heavily in the days before an event rather than only during it.

The counterintuitive part is who it happens to. This is typically seen in recreational athletes rather than elite ones, because they are on the course longer and have more opportunity to overdrink, and women appear to be at greater risk. If you have ever been told there is no such thing as too much water, this is the answer to that.

What to do with all this

Drink to thirst, and check your urine colour rather than a total. Thirst is a functioning mechanism in healthy adults, and the reference bodies describe it as the thing that keeps daily intake where it needs to be.

Use a target as a rough floor if you find one useful, not as a quota to hit. Somewhere around 2 litres of drinks for a woman and 2.5 to 3 for a man is consistent with both sets of reference values, and being under it on a quiet day is not a failure.

Add for heat and for training, and use what you actually lose rather than a formula. Weighing yourself before and after a long session tells you more than any equation, because sweat rates vary between people by a factor of eight.

And if you are fasting, water, black coffee and plain tea all count and none of them break it. The article on what breaks a fast covers where the real edges are.