The 35ml per kg rule
This calculator uses the practical rule most cited by dietitians: roughly 35ml of water per kilogram of body weight per day, as a starting point for a healthy, sedentary adult. To that base value, we add an adjustment for physical activity (sweating during exercise increases fluid replacement needs) and for hot/humid climate (which increases fluid loss through sweat even at rest).
Does this include only plain water?
No — the body's total fluid needs are also partly met by other beverages and by the water content of food (fruits and vegetables, for example, have high water content). The value calculated here is a reference for water consumed as the main beverage, not a rigid limit.
People with kidney, heart or other conditions requiring fluid restriction should follow specific medical guidance instead of this general estimate.
Frequently asked questions
Is drinking too much water a problem?
Yes — although rare, it's possible to cause hyponatremia (dangerous dilution of blood sodium) by drinking amounts well above your needs in a short period, especially common in endurance athletes who drink excess water during long events without replacing electrolytes.
Do coffee and tea count toward daily hydration?
Yes, mostly — caffeine's mild diuretic effect is much smaller than popularly believed, and caffeinated beverages in moderation do contribute to overall hydration, though plain water remains the most recommended option.