# How to Calculate Your Body Mass Index (BMI)

Medically reviewed by Leigh Ann Anderson, PharmD. Last updated on June 14, 2022.

The terms “overweight” and “obese” have specific definitions in healthcare. Overweight and obese are both terms for a range of weight that is greater than what is considered healthy for a given height. Calculating you body mass index (BMI) can provide even more information about your health risks.

BMI is calculated the same way for both adults and children. However, children and teens under 20 years of age, the interpretation of BMI depends upon age and sex, and is interpreted based on percentile charts, usually by your pediatrician.

## What is BMI?

For adults, overweight and obesity ranges are determined by using weight and height to calculate a number called the body mass index (BMI). BMI is used because, for most people, it correlates with their amount of body fat.

BMI is an indirect measure of body fat, but is more convenient than some direct measures of body fat, such as underwater weighing, or dual energy x-ray absorptiometry. BMI is also important because the use of many weight loss drugs are based on a whether a person has reached a certain BMI.

Factors used to determine if someone is at risk for weight-related diseases include:

• BMI
• Waist circumference
• Risk factors for diseases and conditions associated with obesity

Together, these factors are predictors of obesity-related diseases, such as:

The National Heart, Lung, and Blood Institute guidelines recommend that these predictors should be used to assess weight-related health risk.

People with increased muscularity may have a high BMI not necessarily due to excessive body fat, and these BMIs would typically fall in overweight range. However, muscle builders with BMIs in the obese range will usually have a large percentage of body fat.

• Multiply weight in pounds by 703
• Divide that answer by height in inches
• Divide that answer by height in inches again

## BMI Risk Categories

BMI Category
Below 18.5 Underweight
Between 18.5 – 24.9 Normal
Between 25 – 29.9 Overweight
Above 30 Obesity

Obesity may be subdivided into categories:

• Class 1: BMI of 30 to < 35
• Class 2: BMI of 35 to < 40
• Class 3: BMI of 40 or higher. Class 3 obesity is sometimes categorized as “extreme” or “severe” obesity.

For more information about interpretation of BMI for overweight or obese children and teens, visit Childhood Obesity: A U.S. Epidemic

The CDC also has a BMI calculator you can use. Learn more about BMI in children and teens here.

## Sources

• Centers for Disease Control and Prevention (CDC). Overweight and Obesity. Accessed March 30, 2020 at cdc.gov/obesity/adult/defining.html