How to Calculate Your Body Mass Index (BMI)

Medically reviewed by Leigh Ann Anderson, PharmD. Last updated on July 29, 2024.

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, but in children and teens under 20 years of age, the interpretation of BMI depends upon age and sex. BMI in children is interpreted based on percentile charts, typically done by your child's 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, and 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 most 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.

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

In some cases, BMI is not always the best way to describe an individual’s weight-related health. 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 larger percentage of body fat.

BMI may also underestimate body fat in older people or others who have lost muscle.

What health risks are associated with a high BMI?

A higher BMI may mean you are more at risk for certain conditions, such as:

• high blood pressure
• high cholesterol
• type 2 diabetes
• gallstones
• mental health disorders like depression, anxiety, or eating disorders
• osteoarthritis
• sleep apnea or other breathing disorders
• certain types of cancer, like colon, breast or gallbladder

It's always best to assess your BMI in conjunction with your healthcare provider, who can look at your medical history, perform a physical and discuss your health concerns directly. Many people who do not have a high BMI can still at risk for health conditions; conversely, people with a high BMI may not be develop these medical problems. 

The good news is even a small weight loss (about 10%) will help you lower your risk of developing some weight-related diseases.

BMI Risk Categories

Obesity may be subdivided into categories:

• Class 1 Obesity: BMI of 30 to < 35
• Class 2 Obesity: BMI of 35 to < 40
• Class 3 Obesity: BMI of 40 or higher. Formerly known as "morbid" obesity.

How do I calculate my BMI?

To calculate your BMI:

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

For example, a person who weighs 165 lb. and is 5 feet and 6 inches tall (66 inches) would calculate their BMI in the following way:

1. 165 (lb) x 703 = 115,995
2. 115,995 / 66 (height in inches) = 1757.50
3. 1757.50 / 66 (height in inches) = 26.63

Their BMI would be 26.63, which is in the "overweight" risk category (see above).

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.

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

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