How much you should eat depends on how active you are. If you eat more calories than your body uses, you gain weight.

What are calories?
Calories are a way to count how much energy is in food. The energy you get from food helps you do the things you need to do each day. Try to choose foods that have a lot of the nutrients you need, but not many calories.

Just counting calories is not enough for making smart choices. Think about this: a medium banana, 1 cup of flaked cereal, 11⁄2 cups of cooked spinach, 1 tablespoon of peanut butter, or 1 cup of 1% milk all have roughly the same number of calories. But, the foods are different in many ways. Some have more nutrients than others do. For example, milk gives you more calcium than a banana, and peanut butter gives you more protein than cereal. Some foods can make you feel more full than others.