'Eight glasses a day' has never been backed by a single solid study. Real water needs depend on you, not a slogan.
This used to be a back-of-the-envelope job. Now it is one tap, but the logic has not changed. 'Eight glasses a day' has never been backed by a single solid study. Real water needs depend on you, not a slogan.
Daily water (ml) ≈ body weight (kg) × 30–35 ml, adjusted for climate and activity
It originated in a 1945 US recommendation that included water from food. Marketing turned it into a standalone target.
20–30% of daily water intake comes from fruit, vegetables, soup and yoghurt. Pure water need is lower than total fluid need.
Pale yellow urine, no headaches by mid-afternoon, normal energy. Dark yellow or strong-smelling urine = drink up.
Athletes drinking many litres in short windows can dilute sodium (hyponatremia). Match intake to losses, not to a target.
Does coffee dehydrate me? No — for habitual drinkers, coffee contributes to fluid intake. Diuretic effect is mild.
Should I drink more in winter? Slightly less than summer, but indoor heating dries the air. Listen to thirst.
If you only remember one thing from this post, make it that the inputs matter more than the calculator. Try the How Much Water Do You Really Need? The 8-Glasses Rule Is Wrong linked above to put these numbers to work right now.