How it works
This calculator uses a common rule of thumb: about half an ounce of fluid per pound of body weight (roughly 33 ml per kg), plus about 12 oz (355 ml) for every 30 minutes of exercise and an extra half liter in hot or humid weather.
For comparison, the US National Academies set adequate total water intake at about 3.7 liters a day for men and 2.7 liters for women, including the roughly 20% that typically comes from food. Coffee, tea, milk and other drinks all count toward your fluids.
Needs rise with heat, altitude, illness, pregnancy and breastfeeding. Pale yellow urine and rarely feeling thirsty are practical signs you are drinking enough. Results are estimates, not medical advice.
Formula
Water (fl oz) = 0.5 × weight (lb) + 12 × (exercise minutes ÷ 30) + 17 if hot
Questions people ask
Do I really need 8 glasses of water a day?
The 8×8 rule (64 oz) is a memorable starting point but is not based on strong evidence. Needs scale with body size, activity and climate, and food and other drinks supply a good share of fluid.
Does coffee count toward water intake?
Yes. Caffeine has a mild diuretic effect, but studies show moderate coffee and tea still contribute to net hydration.
Can you drink too much water?
Yes, though it is rare. Drinking very large amounts quickly, especially during long endurance events, can dilute blood sodium (hyponatremia). Spread intake through the day and use electrolytes for long, sweaty sessions.