How Much Water Do You Need?
The '8 glasses a day' rule (8×8: eight 8-oz glasses = 64 oz/day) has no scientific basis. It appears to have originated from a 1945 US Food and Nutrition Board recommendation that was widely misinterpreted — the original recommendation actually noted that most of this water need is met by food.
The most current evidence-based guidance comes from the National Academies of Sciences (2004): Men: ~3.7 liters (125 oz) total water per day from all sources. Women: ~2.7 liters (91 oz) total water per day. Roughly 20% of this comes from food (fruits, vegetables, coffee/tea); the remaining 80% from beverages.
Personalized water needs depend heavily on body weight (larger bodies need more), activity level (you can sweat 0.5–2 liters per hour during intense exercise), climate and humidity, and altitude. Our calculator uses the most common evidence-based formula: approximately 35ml per kilogram of body weight per day, adjusted for activity.