The ‘8 glasses a day’ rule is repeated so often it’s treated as fact, but it isn’t based on solid individual science — needs vary a lot by body size, activity, and climate.
A better signal than a fixed number
Urine colour is a simple, practical gauge — pale yellow generally indicates adequate hydration, while consistently dark urine (not accounting for supplements) suggests you need more fluids.
Training changes the picture
Sweat losses during exercise, especially in heat, can be significant. Drinking to thirst during moderate activity is usually sufficient, but longer or hotter sessions may call for deliberate fluid replacement.
Food counts too
A meaningful portion of daily fluid intake comes from food, particularly fruits and vegetables — hydration isn’t only about what you drink.
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