How much water should you drink, and where did eight glasses come from?
There is no single correct number. Needs change with body size, heat, activity and what you eat. A practical starting point is about 30 to 35 ml per kilogram of body weight, adjusted upward for heat and training. Pale yellow urine is a better daily check than any target, and the famous eight glasses has no clear scientific source.
Almost every rule about water is repeated more confidently than the evidence behind it warrants, starting with the most famous one.
Where did the eight glasses come from?
Nobody can point to a study. The most commonly traced origin is a 1945 US Food and Nutrition Board recommendation of roughly 2.5 litres a day — which included, in the very next sentence, the note that most of that quantity comes from prepared food. The sentence about food dropped away and the number kept travelling.
That does not make eight glasses wrong. It makes it an unsourced round number that has been treated as a measurement for eighty years, which is a different problem.
What should you base the amount on instead?
Body size, then conditions. Around 30 to 35 ml per kilogram of body weight a day is a reasonable starting estimate — about 2.1 to 2.5 litres for a 70 kg person — and it is a starting point, not a target to hit.
Then adjust for the things that actually change the requirement: heat and humidity, how much you sweat, how much you train, illness, and how much water your food carries. Someone eating a lot of fruit and soup needs less from a glass than someone eating mostly dry food.
What is a better indicator than a number?
The colour of your urine. Pale yellow, roughly the colour of light straw, is the target. Clear all day generally means you are drinking more than you need. Dark yellow means drink.
It is a better signal than a litre target because it responds to today's conditions rather than to an average. It is also free, immediate, and cannot be fooled by a spreadsheet.
Two caveats worth knowing: vitamin B2 in a multivitamin turns urine bright yellow regardless of hydration, and some medications colour it too. If either applies, use thirst and how you feel instead.
Do tea and coffee count?
Yes. The belief that caffeine dehydrates you comes from studies in people who do not normally drink it, given large doses. In habitual drinkers, the diuretic effect is small and the fluid in the cup more than covers it.
A cup of tea or coffee contributes to your daily fluid. Whether it contributes to your sleep is a separate question, and that one has a clearer answer.
Does drinking more water help you lose weight?
A little, and by an unglamorous mechanism: a glass before a meal takes up room and slightly reduces how much is eaten. Some trials find a small effect and others do not, and the size of it is nowhere near what the claim usually implies.
Replacing sweetened drinks with water, on the other hand, has a large and reliable effect — but that is calories removed, not water added. The two get talked about as if they were the same thing, and only one of them does much.
Reading something you can act on beats reading something you cannot.