How much water do you need?

The eight-glasses rule has no good evidential basis, which does not mean hydration is unimportant — it means the number was never the useful part.

Moves your needs a lot

• Training volume and sweat rate
• Heat and humidity
• Bodyweight
• Caffeine and alcohol
• Illness

Moves it very little

• A fixed glass count
• The time of day
• Whether it is plain water

Where eight glasses came from

Most likely a misreading of mid-century guidance that estimated total daily water needs — and then noted that much of it comes from food. The food half got dropped and the slogan survived.

Total intake includes what is in your food, tea, coffee and everything else. Those all count.

What actually determines your needs

Sweat rate above all. An hour of hard training in a hot room can cost more fluid than a whole sedentary day.

Body size, climate, altitude and illness all move it substantially.

This is why a single number for everyone was never going to work.

A more useful rule

Urine colour is a decent everyday proxy: pale straw is fine, consistently dark suggests drinking more.

Drink to thirst in ordinary conditions — thirst is a reasonably good regulator in healthy adults — and deliberately more around training in heat.

Weighing before and after a long session tells you your own sweat rate: each kilogram lost is roughly a litre of fluid.

On overdoing it

Drinking enormous amounts of plain water quickly can dilute blood sodium, which is dangerous. It is rare, and it happens in endurance events rather than in daily life — but "more is always better" is not true here either.

Questions