It’s no secret that a healthy body weight is an important factor in overall health. It’s also an important tool for managing and preventing chronic illnesses. So in order to determine whether or not you are at your optimal weight, your doctor may use the body mass index (BMI) chart.

BMI simply takes into account the relationship between your weight and your height, and then gives you a number that represents your overall body mass index. If your number is too high, you’re not at an ideal weight for your height. If it’s too low, you likely need to gain weight in order to be the healthiest you can be. Here’s how the BMI numbers break out.

• Less than 18.5 BMI is considered underweight
• 18.5 to 24.9 BMI is considered healthy
• 25 to 29.9 BMI is considered overweight
• 30 BMI or higher is considered obese

See the chart below to calculate your BMI. If you find that your BMI is somewhere above or below the ideal, here’s a look at what you can do.

For a high BMI:

If your BMI is 25-30 or above, it may be time to get in shape. As always, the best way to lose a few of those extra pounds is to eat healthy and get active. Start out slowly by replacing one snack of chips or sweets with a piece of fruit each day. Then avoid heading straight to the couch in the evening and take an after-dinner walk.

For a low BMI:

If your BMI is 18.5 or lower, it’s time to talk to your doctor. Your body may not be absorbing nutrients the way it should be, or you may have an under lying condition that’s causing weight loss. Your doctor will help you figure out what’s going on and work with you to get your health back on track.

Your body mass index is one part of your greater health story. If you’re concerned about your number, it’s best to talk to your doctor. Your doctor can help you determine whether or not there’s a problem and work with you to create an action plan.

What’s not included in BMI

BMI does not account for bone density and muscle mass—both of which add weight. So if you’re in exceptional shape with lots of muscle, your BMI may skew a little higher.