Understanding Body Mass Index

Reviewed by Sarah Klemm, RD, CD
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Body Mass Index, also known as BMI, is a ratio of your weight to height, and it is used as an indicator of body fat. BMI is an inexpensive alternative to direct measurements of body fat, such as underwater weighing or bioelectrical impedance, but it is only one of many factors that is used in evaluating your health status.

Calculating Your BMI

You can calculate your BMI with this formula:

Weight (in pounds) / [Height (in inches) x Height (in inches)] x 703.

The Centers for Disease Control and Prevention use BMI to define terms including overweight and obese:

  • Underweight: BMI below 18.5
  • Normal weight: 18.5 to 24.9
  • Overweight: 25.0 to 29.9
  • Obese: 30.0 and above.

BMI and Your Health

People with a BMI outside of the normal weight range may be at the greatest health risk. Those who are considered to be underweight may not be getting enough calories or receiving adequate nutrients, putting them at risk for certain health conditions. Whereas individuals with an overweight or obese BMI tend to be at increased risk for high cholesterol, diabetes, heart disease, sleep apnea and some types of cancer.

However, BMI is only one factor in your overall health. For example, if your BMI falls into the normal weight category, you are still at a higher risk of health problems if you:

  • Smoke
  • Do not participate in regular physical activity
  • Choose foods low in vitamins and minerals and high in saturated fats and added sugars.

If your BMI is in the overweight category, you will have a lower overall health risk if you:

  • Get regular physical activity
  • Have blood pressure, blood sugar and cholesterol levels within normal limits.

BMI is just one aspect of your health and should be discussed with your health care provider. Together, you can decide if other assessments need to be done and whether lifestyle changes such as eating better and moving more are enough to improve your health.

BMI Measurements in Children and Teens

While BMI calculations for children and teens use the same formula as adults, criteria used to define obesity and overweight are different for young people because of factors including body fat differences between boys and girls and variations in body fat at different ages. Visit the BMI Percentile Calculator for Child and Teens to determine your child's BMI.

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