How Much Water Should You Drink Per Day?
The 'eight glasses a day' rule is outdated. Here is what science actually says about hydration based on body size and activity.
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'Drink eight glasses of water a day' is one of the most repeated health rules, and one of the least supported by evidence. Real water needs depend on body size, activity, climate and diet.
A better rule
The Institute of Medicine recommends about 3.7 liters (125 oz) for men and 2.7 liters (91 oz) for women per day. This includes water from food and other beverages, not just plain water.
Body weight rule
A simple practical rule: drink 30 to 35 ml per kilogram of body weight. A 70 kg person needs 2.1 to 2.5 liters per day from all sources. A 100 kg person needs 3 to 3.5 liters.
Adjust for activity and climate
Add 0.5 to 1 liter per hour of intense exercise. Add 10-20% in hot or humid weather. Athletes and outdoor workers may need 5-6 liters in extreme conditions.
Signs you are well-hydrated
- Pale yellow urine (dark yellow means drink more) - Rarely thirsty - Energy levels stable - No headache from dehydration
Foods count too
About 20% of daily water comes from food. Fruits and vegetables are 80-95% water. Soup, dairy, coffee and tea also count toward your daily intake.
Can you drink too much?
Yes. Overhydration (hyponatremia) is rare but serious. Avoid drinking more than a liter per hour for extended periods unless replacing electrolyte losses.
Practical tips
- Drink a glass when you wake up - Keep a water bottle visible - Drink before, during and after exercise - Add lemon or fruit if plain water is boring
Use our health tools
Calculate your BMR to know your basic energy needs. Combine with our BMI Calculator and Calorie Calculator for a complete picture of your daily needs.