“Drink 8 glasses of water a day.” You’ve heard this a thousand times. There’s no scientific basis for it. The number was loosely derived from a 1945 US government report that recommended 2.5 liters of daily water intake, but noted that most of this comes from food. That last part got dropped, and a myth was born.

What the Research Actually Says

The Institute of Medicine suggests:

• Men: about 3.7 liters total water per day
• Women: about 2.7 liters total water per day

“Total water” includes water from food (which accounts for roughly 20-30% of intake). So the actual drinking requirement is lower.

But these are averages for temperate climates with moderate activity. Your needs vary based on several factors.

What Changes Your Water Needs

Body weight

Larger bodies need more water. A rough formula: 30-35 ml per kg of body weight.

A 70 kg person: 2.1 to 2.45 liters. A 90 kg person: 2.7 to 3.15 liters.

Physical activity

You lose 0.5 to 1 liter of water per hour of moderate exercise through sweat. Intense exercise in heat can push this to 2+ liters per hour.

Climate

Hot and humid weather increases sweat losses. Dry climates increase water loss through respiration and skin. Both require extra intake.

Diet

High-sodium food increases water needs. High-water-content foods (fruits, vegetables, soups) reduce how much you need to drink separately. A person eating lots of watermelon and cucumber needs less drinking water than someone eating mostly dry, processed food.

Altitude

Above 2,500 meters, your body loses more water through increased respiration and urine output. High-altitude trekkers should increase intake by 0.5-1 liter.

The “8 Glasses” Verdict

For a sedentary person in a temperate climate eating a balanced diet, 8 glasses (about 2 liters) is… actually fine. Not because the rule is scientifically derived, but because it happens to land in the right ballpark for many people. It’s an okay default, just not a universal truth.

The Best Indicator: Urine Color

Forget counting glasses. Your body has a built-in hydration meter.

• Pale yellow: well hydrated
• Light yellow: adequately hydrated
• Dark yellow: drink more water
• Clear/colorless: you’re overhydrating (yes, this is possible)

Check first thing in the morning. That’s your baseline hydration level.

Can You Drink Too Much?

Yes. Hyponatremia (dangerously low blood sodium from excessive water intake) is rare but real. It most commonly affects endurance athletes who drink far more than they sweat. For most people, the kidneys handle excess water efficiently, but there’s no benefit to forcing down 5+ liters when your body doesn’t need it.

Practical Guidelines

1. Start with 30-35 ml per kg of body weight as a baseline
2. Add 500 ml for every 30 minutes of exercise
3. Add 500 ml in hot or dry climates
4. Drink when thirsty (thirst works well in healthy adults)
5. Check urine color once a day
6. Water, tea, coffee, and milk all count toward intake
7. Coffee is not dehydrating in normal amounts (the diuretic effect is mild and offset by the water content)