BodyCalThe 8-glasses myth
The famous 'eight 8-ounce glasses' rule has no solid scientific origin — it appears to be a misreading of a 1945 nutrition board recommendation that already included water from food. Your real needs depend on your body size, how much you sweat, and your environment.
A better starting point: roughly 35 ml of water per kilogram of body weight per day. A 70 kg adult lands at about 2.4 litres, close to the European Food Safety Authority's adequate intake guidance of 2.0 L/day for women and 2.5 L/day for men from all sources.
Adjust for exercise and heat
Sweat is the biggest variable. An hour of moderate exercise can cost 0.5–1 litre of fluid; intense training in heat can double that. Hot or humid climates raise baseline losses even at rest, which is why our water intake calculator adds explicit adjustments for both activity level and climate.
Spread intake through the day rather than chugging — your kidneys can only process so much at once, and steady sipping maintains hydration far better than catch-up gulping.
Signs you're getting it right
Pale-yellow urine, steady energy, and rarely feeling parched are the practical markers of good hydration. Dark urine, afternoon headaches, and persistent fatigue often point the other way.
Remember that food typically contributes 20–30% of your fluid intake, and coffee and tea count toward your total despite the old caffeine-dehydration myth.