We often hear the claim that we should drink "eight glasses of water a day"—a so-called golden rule for staying healthy. But in recent years, this idea has come under scrutiny, with some experts calling it misleading. So, is it really necessary to drink exactly eight glasses of water daily? How much water do we actually need to stay healthy? Let’s get to the bottom of it.

The Origin of the “Eight Glasses” Rule：

The “eight glasses a day” rule originated in 1945, when the U.S. Food and Nutrition Board recommended drinking one milliliter of water for every calorie consumed. For someone eating 2,000 calories a day, that amounts to about 2 liters—or roughly eight 240 ml glasses of water.

Individual Differences Matter：

While “eight glasses” may serve as a general guideline, water needs can vary significantly from person to person due to factors such as diet, physical activity, and environmental conditions.

1. Water from Food

Our water intake doesn’t come solely from drinking. Many foods—especially fruits, vegetables, and soups—contain a high amount of water. Depending on dietary habits, the proportion of water we get from food can vary widely. For instance, traditional Chinese cooking methods often retain or even add water to dishes, increasing the amount of water consumed through food. So if your meals are rich in water-containing foods, you might not need to drink as much.

2. Caloric Intake and Water Needs

Not everyone consumes the same amount of energy each day, which affects water requirements. High-protein diets, for example, require more water to aid in metabolism, while ketogenic diets (low in carbs) may also require extra hydration due to glycogen’s association with water in the body. Even at the same 2,000 calorie level, variations in diet composition can alter your water needs.

3. Sweating and Excretion

Water loss varies from person to person. Those who sweat more—either from exercise or hot climates—will naturally need more water. People living in hot areas or who are physically active will generally require greater hydration than those with more sedentary lifestyles in cooler environments.

What Does Proper Hydration Look Like?

1. Adjust for Physical Activity

If you’re engaging in intense exercise or exposed to high temperatures, your body loses water more quickly through sweat. In these cases, you should increase your water intake accordingly to stay hydrated.

2. Tailor to Personal Needs

Age, body weight, and gender all play roles in determining how much water you need. Younger people and those with higher body weight often need more water. Pregnant and breastfeeding women also require increased hydration. Meanwhile, elderly individuals or those with lower body mass may need slightly less.

3. Consider Your Diet

A diet high in processed, low-moisture foods—like crackers or chips—will likely require you to drink more water. Since these foods contain minimal water, your external hydration needs increase.

4. Don’t Wait Until You’re Thirsty

To maintain optimal hydration, it’s better to drink water regularly throughout the day rather than waiting until you feel thirsty.

5. Special Conditions: Illness or Environment

When you're sick—with fever, vomiting, or diarrhea—your body loses more fluids and requires extra water. Likewise, if you live in particularly dry, cold, or hot climates, you should increase your water intake accordingly.

Conclusion

The “eight glasses a day” rule is a useful general recommendation for healthy individuals, but it isn’t a strict standard that everyone must follow. Water needs vary based on individual health, habits, and environmental factors. The healthiest approach is to drink water according to your body’s needs, maintain hydration balance, and establish regular drinking habits—rather than blindly adhering to a fixed number.