Body Mass Index (BMI)

Medically Reviewed By William C. Lloyd III, MD, FACS
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What is BMI?

BMI stands for body mass index. It is a ratio of body weight to height and is an estimate of body fat. It is better than just measuring weight because it takes your height into account. While it is not a direct measure of body fat, it does correlate well with methods that are. These methods include skinfold thickness, bioelectrical impedance, underwater weighing, and DXA (dual X-ray absorptiometry). BMI gives doctors a quick and inexpensive way to assess a person’s body fat.

What does BMI mean for my health?

Estimating your body fat helps doctors determine your risk of health problems. But keep in mind that BMI is just one marker doctors use to assess your overall health. The results of a physical exam and the information in your medical history are also important for determining your risk of disease.

Generally, high BMI numbers put you at risk for diseases, disorders and conditions associated with excess weight including:

Knowing your BMI can help you take control of your health. If your BMI is high, your doctor may recommend losing weight. Shedding 5 to 10% of your body weight will decrease your risk of developing these health hazards.

What is my BMI?

To arrive at a BMI, you divide your weight in kilograms (kg) by the square of your height in meters (m2). The result is reported as kg/m2. You can use a body mass index calculator and plug in your numbers. Usually, you can enter your information in pounds and inches and the BMI calculator will convert it for you. There are separate calculators for adults and children.

What BMI is healthy and what BMI is overweight?

When determining what your BMI means, it’s important to keep some limitations in mind. Remember that BMI is just an estimate of body fat. It does not directly measure body fat. Because of this, it can overestimate body fat in certain people. This includes athletes and other adults who weigh a lot due to high lean muscle mass. It can do the opposite and underestimate body fat in people who do not have a lot of muscle mass. This includes the elderly. It’s best to discuss your BMI result with your doctor to put it in the proper perspective.

Adult BMI

BMI interpretation in adults is fairly simple. BMI calculators for adults only require your height and weight. Your result will fall into one of the following standard weight status categories:

  • Underweight BMI is less than 18.5 kg/m2.

  • Normal or healthy weight BMI is between 18.5 and 24.9 kg/m2.

  • Overweight BMI is between 25 and 29.9 kg/m2.

  • Obese BMI is equal to or greater than 30 kg/m2.

  • Morbid obesity BMI is equal to or greater than 40 kg/m2. Other names for this are extreme or severe obesity.

Child and Teen BMI

Determining BMI in children and teens is more complicated than adults. In addition to height and weight, you must also consider their age and sex. Doctors plot this information on growth charts. You commonly see these growth charts at your child’s wellness checkup. The chart divides children into percentiles. Weight status categories are based on these percentiles as follows:

  • Underweight is less than the 5th percentile.

  • Normal or healthy weight is the 5th percentile and to less than the 85th percentile.

  • Overweight is the 85th percentile to less than the 95th percentile.

  • Obese is 95th percentile or greater.

Starting at age 2, doctors will use BMI to screen for overweight and obesity in children. Like adults, BMI is just one data point doctors use to assess health and health risks in children. Doctors evaluate BMI in the context of diet, physical activity, family history, and other health screenings. If necessary, more accurate and direct measures of body fat, such as skinfold thickness, may be necessary.

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Medical Reviewer: William C. Lloyd III, MD, FACS
Last Review Date: 2020 Aug 29
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THIS TOOL DOES NOT PROVIDE MEDICAL ADVICE. It is intended for informational purposes only. It is not a substitute for professional medical advice, diagnosis or treatment. Never ignore professional medical advice in seeking treatment because of something you have read on the site. If you think you may have a medical emergency, immediately call your doctor or dial 911.
  1. About Adult BMI. Centers for Disease Control and Prevention. https://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/index.html
  2. About Child and Teen BMI. Centers for Disease Control and Prevention. https://www.cdc.gov/healthyweight/assessing/bmi/childrens_bmi/about_childrens_bmi.html
  3. Assessing Your Weight and Health Risk. National Heart, Lung, and Blood Institute. https://www.nhlbi.nih.gov/health/educational/lose_wt/risk.htm
  4. Body Mass Index (BMI). Centers for Disease Control and Prevention. https://www.cdc.gov/healthyweight/assessing/bmi/index.html
  5. Calculate Your Body Mass Index. National Heart, Lung, and Blood Institute. https://www.nhlbi.nih.gov/health/educational/lose_wt/BMI/bmicalc.htm
  6. Defining Adult Overweight and Obesity. Centers for Disease Control and Prevention. https://www.cdc.gov/obesity/adult/defining.html