What Is Body Mass Index (BMI)?

Body mass index (BMI) is an estimate of body fat that is based on your weight and height. This calculation helps determine whether you are underweight, at a healthy weight, overweight, or obese.

BMI can be used to assess your risk for developing certain health problems. For example, people in higher weight categories are considered to be at greater risk for high blood pressure and high cholesterol.

However, the calculation has its limitations. Namely, BMI doesn't take into account age, assigned sex, race, or muscle mass.

This article discusses BMI, how it is calculated, its pros and cons, and alternatives for assessing potential health risks.

How BMI Is Calculated

BMI is calculated by dividing your body weight in kilograms by your height in meters squared.

• Formula: weight (kg) / [height (m)]²

BMI can also be calculated by dividing your weight in pounds by your height in inches squared, then multiplying the answer by 703.

• Formula: weight (lb) / [height (in)]² x 703

You may not have all of these numbers at the ready. Online BMI calculators can do the work for you.

BMI Scores

Body mass index scores are broken down into the following categories:

• Underweight: Less than 18.5
• Normal: 18.5 to 24.9
• Overweight: 25 to 29.9
• Obese: 30 and above

Obesity is sometimes broken down into additional categories:

• Class 1: BMI 30 to <35
• Class 2: BMI 35 to <40
• Class 3 (severe obesity): BMI of 40 or higher
  

BMI Chart for Children

BMI Chart for Adults

Body Mass Index and Health

Excess body fat has been linked to increased risk of a number of health issues. BMI scores in the overweight and obese categories can indicate that a person is at higher risk of certain diseases, such as:

• Heart disease
• High blood pressure
• Cancer
• Osteoarthritis
• Type 2 diabetes
• Gallstones
• Sleep apnea
• High cholesterol
• Liver problems

Pros and Cons of BMI

On the plus side, BMI is a simple way to estimate body fat. It’s convenient, inexpensive, and can be used routinely. As a result, it’s an easy way to identify someone who may have a health risk worth looking into. It’s also used universally by medical professionals in research around the world.

However, it also has significant limitations. To start with, it can't be used to diagnose health issues—just identify possible red flags.

It can also be a false alarm for a significant number of people due to the following factors that BMI does not account for.

Muscle Mass

Muscle and bone are denser than fat. Since the only measures used to calculate BMI are weight and height, it can overestimate body fat in athletes or individuals with lots of muscle mass.

It can also underestimate body fat in people with very little muscle mass.

Race

BMI does not account for a person's race, which can influence weight-related health risks and body composition, the amount of body weight that is made up of fat versus lean muscle tissue.

On average, for example, Asians and South Asians have greater proportional body fat than Europeans at the same weight, while Blacks have less body fat and more muscle at the same weight as compared to Europeans.

For this reason, some experts now argue for tailored cutoffs for specific groups.

Assigned Sex

Those born female tend to have more body fat in comparison to men. BMI doesn’t account for this difference. Therefore, a woman may register in a “safe” category of BMI but still have levels of body fat that can represent a health risk.

Age

Normal BMI ranges may not accurately predict health risks for the young and old. For example, in the elderly, a BMI below 23—which is in the so-called normal range for adults—is associated with higher mortality, while being in the “overweight” range is not.

How Fat Is Distributed

Evidence suggests that where body fat is located also matters. Those who carry weight around the middle—what's called an “apple” body shape—have higher health risks than people with a "pear" shape (i.e., those who accumulate weight in the hips and thighs).

Alternatives to BMI

Some have called for retiring BMI because of these issues. While its disadvantages are widely accepted, BMI remains in use partly because it’s simple to use.

When desired or considered necessary, other methods for measuring body fat or assessing your health risk can be used in place of, or in addition to, BMI.

These include:

• Waist circumference: Fat that is carried around the middle of your body can increase your risk of type 2 diabetes and heart disease. Risk increases for women with waist measurements greater than 35 inches, and for men with waist measurements greater than 40 inches. A number of experts now suggest waist circumference is a better measure for all people, but may be particularly helpful in evaluating health risks for people of color.
• Waist-to-hip ratio: Dividing your waist circumference by your hip circumference can provide information about potential health risks. A measurement of 0.9 or more for women and 1.0 or more for men indicates a higher risk.
• Skin-fold measurements: Body fat percentages can be estimated through the use of calipers—a tool that measures the thickness of the skin. Measurements are taken in multiple places on the body, then plugged into a formula.
• Smart scale: Body fat can be estimated with certain "smart" bathroom scales. These scales measure body fat by sending a harmless electrical current through your body. Note that the technology smart scales use requires a constant level of hydration in the body. Try to weigh
  yourself at the same time of day, when you know you’ll be consistently hydrated, to avoid inaccurate measurements.

More sophisticated methods of measuring body fat and composition also exist. However, most are not available in standard medical offices. Instead, they’re used in specialist or research settings.

Examples include:

• Underwater weighing: With this method, also called hydrostatic weighing or densitometry, you’re weighed in air and subsequently in water. The two measurements are then compared to calculate body fat.
• Air displacement plethysmography (ADP): ADP uses air to estimate your body fat percentage based on the density of your body.
• Dual energy X-ray absorptiometry (DEXA) and MRI: These sophisticated imaging technologies are used to examine body composition, including fat, muscle mass, and bone density.
  

Normal BMI and Good Health

It's possible that you could have a normal BMI but still have heightened health risks.

For example, maybe your BMI is in the normal range, but you are not physically active, your diet is poor, you have a family history of disease, or you smoke—any of which can affect your risk of a host of health concerns.

A senior adult could have a normal BMI, but have significant health issues like hardening of the arteries (atherosclerosis).

And, as discussed, it's possible that your BMI could be in the healthy range, but your race, age, sex, or another factor make that measurement less accurate in your case.

Consider BMI for what it is—an estimate, not a measurement, and just one piece of information that can help paint the picture of your overall health. Your healthcare provider can help you better understand how your BMI fits in.

Summary

BMI provides an estimate of your body fat, which can influence your risk of developing diseases. Calculating BMI is quick, and free BMI calculators are available online.

However, BMI has a number of limitations. This measure does not take into account your age, race, sex, or fitness level.

In addition, multiple factors need to be considered when determining what a healthy weight is for you. BMI can provide one small piece of information, but it should not be the only resource used when deciding how much you should weigh to stay well.