What’s Your Body Mass Index (BMI)?

Written By:Stacy Simon

BMI is a way to figure out your approximate level of body fat in an easy, inexpensive way. It’s a number based on your weight and height that can help you tell if you are at a healthy weight for your height. In general, the higher the number, the more body fat a person has. BMI is often used to determine if your weight might be putting you at risk for health problems such as heart disease, diabetes, and cancer.

BMI is calculated by dividing a person’s weight in kilograms by the square of their height in meters. Our BMI calculator will do this math for you. The result will tell you which weight-group category you fall into.

  • BMI less than 18.5 = Underweight
  • BMI 18.5 to 24.9 = Normal Weight
  • BMI 25 to 29.9 = Overweight
  • BMI 30 and higher = Obese


For most people, the BMI gives a good estimate of total body fat, but it doesn't work well for everybody. For example, bodybuilders or other muscular people may have a high BMI because of their muscle mass, even though they're not necessarily overweight. The BMI can also underestimate body fat in people who have lost muscle mass, such as some older people.

BMI is calculated using the same formula for children and teens, but the number is interpreted differently and depends on their age and gender. If you have concerns about your child’s height or weight, talk to their medical provider.

Next steps

For most adults, BMI is a convenient way to get an idea of healthy weight ranges. But it is not always the final word in deciding if a person is at a healthy weight. Your doctor might use other factors including waist size, blood sugar levels, and other tests to find out if your weight might pose a health risk.

If your BMI result does show that you are overweight, consider making some lifestyle changes. Studies show that even a small weight loss – 10% of your current weight – lowers the risk of several diseases.

The American Cancer Society medical and editorial content team

Our team is made up of doctors and oncology certified nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.

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