The 8 Glasses Myth
The recommendation to drink "8 glasses of water per day" has no scientific basis. It likely originated from a misinterpretation of a 1945 US Food and Nutrition Board report that recommended 2.5 liters of daily water intake - but also noted that most of this comes from food. The actual research-backed recommendations are more nuanced.
How Much Water Do You Actually Need?
The National Academies of Sciences (2004) Dietary Reference Intakes recommend:
- Men: 3.7 liters (125 oz) of total water per day
- Women: 2.7 liters (91 oz) of total water per day
Critically, this includes water from all sources - drinking water, other beverages (coffee, tea, milk), and food. Approximately 20% of daily water intake comes from food. So the drinking water target is roughly 3.0 liters for men and 2.2 liters for women.
Factors That Increase Water Needs
- Exercise: Add approximately 500ml (17 oz) per hour of moderate exercise. Intense exercise in hot conditions may require 750ml-1 liter per hour.
- Climate: Hot and humid environments increase water loss through sweat. Increase intake by 500ml-1 liter per day in hot weather.
- Body size: Larger bodies need more water. A general formula: 30-35ml per kg of bodyweight.
- Altitude: High altitude increases respiration rate and fluid loss.
The Simplest Hydration Indicator
Your urine color is the easiest way to assess hydration status. Pale yellow (like lemonade) indicates adequate hydration. Dark yellow or amber indicates dehydration. Clear/colorless may indicate overhydration, which is also not ideal.
Calculate Your Target
Use our free Water Intake Calculator for a personalized daily water target based on your weight, activity level, and climate.
