Humans and Water
Compared to the other animals featured in this section, humans seem to be at a disadvantage when it comes to water. We're dependent on the day-to-day availability of fresh, clean water. We can't survive on seawater, we can't live underwater, and we can't capture rainwater on our backs. We lose water through perspiration, urination, and by exhalation--we breathe out water vapor. Only by using our intelligence and ingenuity have humans managed to live in areas where water is relatively scarce, as well as places where it's salty or frozen.
How Much Water Is Inside You?
The rule of thumb is that humans are about 60 percent water by weight. Because of differences in body fat, men carry a little more than that, women a little less.
How Much Water Should I Drink?
Contrary to popular wisdom, you don't need to drink eight glasses of water a day to replenish your body's water supply. The water in food gives you about 20 percent of your daily needs. Beverages like milk or juice help keep you hydrated, too. The best rule of thumb for most people: Drink water whenever you're thirsty.
By the Numbers
- A typical woman needs about 2.7 liters (0.7 U.S. gallons) of water per day.
- A typical man needs about 3.7 liters (one U.S. gallon) per day.