How much water do we really need?

The eight-glass formula doesn't fit for everyone. It depends on your gender, size, and level of activity, other studies note. The Institute of Medicine calls for adult men to drink 13 cups of fluid daily (which totals three liters, or a little more than four tall reusable water bottles) and women to have nine cups (2.2 liters, or about the amount found in three reusable water bottles). That number changes according to lifestyle.

The more active a person is, the more they will need to replace fluids. Larger people, pregnant and nursing women, and those who take dehydrating medications also will likely need to account for that by adding more water to their diet.

If you are active, it's also important to account for your environment when calculating how much fluid you need. If you are exercising in temperatures that are very hot (likely leading you sweat more) or very cold (which can stunt your ability to sense dehydration), are an endurance athlete, or are active in a high-altitude area, you will probably need to up your water intake.