How the water target is estimated
This calculator assumes a widely used rule of thumb — roughly 30–35 mL of water per kilogram of body weight per day — plus extra water for exercise sweat loss. It is not copied from any specific health authority's recommendation.
- 0.033 — L per kg body weight (30–35 mL/kg rule of thumb)
- 0.35 — L added per 30 minutes of exercise
More detail
Why body weight first
Larger bodies carry more water and lose more through normal function, so weight-based guidelines scale better than a flat "8 cups a day" rule. The 30–35 mL/kg range is a common rule-of-thumb starting point, adopted here as an assumption rather than as any published recommendation.
Exercise adds to the target
Sweat losses during exercise can add up quickly, especially in heat. Adding roughly 350 mL per 30 minutes of activity is a simple way to account for that without needing to weigh yourself before and after a workout.
Practical tip. Thirst, urine color, and activity level are better real-time signals than any formula. Treat this number as a starting target and adjust for climate, sweat rate, and how you actually feel.
Frequently asked questions
How much water should I drink a day?
Roughly 30-35 mL per kg of body weight, plus extra for exercise. For a 70 kg person with no exercise, that's about 2.3 L — enter your own weight above for a precise number.
Is the '8 glasses a day' rule accurate?
It's a rough average that ignores body size, so it overestimates for smaller people and underestimates for larger ones. A weight-based formula like the one above scales more sensibly across different body sizes.
Does this include water from food and other drinks?
No — this is a target for water specifically. Food and other beverages contribute additional fluid, so your actual drinking-water need may be somewhat lower depending on your diet.
Should I drink more in hot weather or at altitude?
Yes. Heat, humidity, altitude, and illness all increase fluid loss beyond what this general formula accounts for. Use the result as a baseline and drink more when you're sweating heavily or feeling thirsty.