Water Intake Calculator
A general guideline based on body weight and activity level — not medical advice. Needs vary by climate, health conditions, and individual factors.
How Water Intake Calculator Works
This tool estimates how much water to drink each day using a simple milliliters-per-kilogram-of-body-weight guideline, adjusted upward for how physically active you are. It's a common rule-of-thumb approach, not a personalized medical recommendation.
Formula & Method
The calculator multiplies your body weight in kilograms by a milliliters-per-kg figure that depends on your activity level: 30 ml/kg for sedentary, 35 ml/kg for lightly active, 40 ml/kg for moderately active, 45 ml/kg for very active, and 50 ml/kg for extra active. The result, weight (kg) × ml per kg, is shown in liters and also converted to milliliters, fluid ounces (× 0.033814 per ml), 240 ml cups, and 250 ml glasses.
Worked Example
For a 70 kg person who is lightly active (35 ml/kg): total intake is 70 × 35 = 2,450 ml, which is 2.45 L. That converts to about 82.8 fl oz, roughly 10.2 cups (240 ml each), or 9.8 glasses (250 ml each).
Frequently Asked Questions
- Is this the same as the "8 glasses a day" rule?
- Not exactly -- "8 glasses" is a rough, one-size-fits-all rule of thumb, while this calculator scales the recommendation to your actual body weight and activity level, which is why the result can land above or below 8 glasses depending on your inputs.
- Does this number include water from food and other drinks?
- No -- this estimates total fluid intake from drinking water specifically. In practice, food (especially fruits, vegetables, and soups) and other beverages also contribute to your daily hydration, so your actual drinking-water needs may be somewhat lower than the figure shown.
- Why does activity level change the recommendation so much?
- More physical activity means more fluid lost through sweat and faster breathing, so the ml-per-kg figure increases at each activity tier to account for that extra loss -- from 30 ml/kg at rest up to 50 ml/kg for very intense activity or physically demanding jobs.
- Should I drink more in hot weather?
- Likely yes, but this calculator doesn't factor in climate or temperature -- it only uses weight and activity level. Hot or humid conditions increase fluid loss through sweat beyond what's captured here, so treat the result as a baseline you may need to raise in hot weather.