How Much Water Should I Drink a Day?
"Eight glasses a day" is easy to remember but isn't based on much. Actual needs depend on body size, activity, climate — and on the water you already get from food.
A practical estimate
A common rule of thumb is about 35 mL per kg of body weight, then add:
- about 0.6 L per hour of exercise (more if you sweat heavily);
- about 0.5 L on hot or very dry days.
Example: 70 kg × 35 mL = 2.45 L, plus 45 minutes of exercise (0.45 L) → about 2.9 L of fluids that day. The water intake calculator works it out in liters or ounces.
Official reference values
The European Food Safety Authority (EFSA) sets adequate total water intake at 2.0 L/day for women and 2.5 L/day for men. "Total" includes food, which usually provides about 20% — fruit, vegetables, soups and dairy are mostly water. The US National Academies give higher totals (about 2.7 L and 3.7 L), also including food.
Listen to your body
For healthy adults, thirst and pale yellow urine are reliable everyday guides. Needs rise with fever, vomiting, diarrhea, heat and long workouts.
Too much is possible
Drinking very large amounts in a short time — especially during endurance events — can dangerously lower blood sodium (hyponatremia). Drink to thirst rather than forcing liters.
When this doesn't apply
People with kidney, heart or liver conditions may be told to limit fluids. Follow your doctor's instructions over any calculator. Children, pregnant and breastfeeding women have different needs.
See also: estimate daily energy needs with the calorie calculator.