What Is Body Mass Index (BMI)?

Body mass index, or BMI, is a common measure that helps to determine whether a person is overweight or underweight. This information can be used to assess risk for developing certain health problems. However, BMI has its limitations; it doesn’t take into account a person’s age, race/ethnicity, or muscle mass.

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

What Is Body Mass Index?

Body mass index assigns a number based on a comparison of your weight and height.1 This number then places you in a category to determine whether you are underweight, at a healthy weight, overweight, or obese. Higher weight categories are considered to be at higher risk for certain illnesses, such as high blood pressure and high cholesterol.

How to Calculate BMI

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

If you don’t feel like doing math, online BMI calculators can do the work for you.

Body mass index scores are broken down into several categories:2

  • 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

Body Mass Index and Health

BMI is an estimate of body fat. Excess body fat has been linked to increased risk of several health issues. BMI scores in the overweight and obese categories can indicate that a person is at higher risk of certain diseases, such as:3

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

Pros and Cons of BMI

BMI has its pros and cons. On the plus side, BMI is a relatively simple, inexpensive way to estimate your body fat. However, it has many limitations.

BMI can overestimate body fat in athletes or individuals with a large amount of muscle mass. It can also underestimate body fat in elderly people or in people with very little muscle mass.3

Current BMI measures are largely based on the Caucasian population—BMI does not account for a person’s race or ethnic group, which can influence body composition.4

Alternatives to BMI

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.5 Risk increases for women with waist measurements greater than 35 inches, and for men with waist measurements greater than 40 inches.
  • 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.6
  • 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.7
  • Smart scale:Body fat can be estimated with bathroom scales that have additional features. Smart scales measure body fat by sending a harmless electrical current through your body.

The 7 Best Bathroom Scales

What Is a Healthy Weight?

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 when deciding what you should weigh.

In addition, being a “healthy weight” doesn’t automatically equate to being healthy. Your overall body composition—the amount of body weight that is made up of fat versus lean muscle tissue—is much more important in determining your health risks.

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 several limitations. This measure does not take into account your age, race, ethnicity, or health history.

A Word From Verywell

While BMI provides one specific piece of information—a comparison of your weight to your height—it should not be used to determine your ideal weight. Body weight is only one piece of a much bigger puzzle. Talk to your doctor for tips on living a healthy lifestyle.

Frequently Asked Questions

  • Is gender a factor in calculating BMI? Adult BMI measurements are not based on gender.
  • Is BMI an accurate way to measure body fat? BMI is an estimate of body fat, but it is not an accurate way to determine specific body fat percentages.
  • Is there a better measurement to use instead of BMI? Assessing body composition—amount of body fat versus lean muscle tissue—is a more accurate way than BMI to determine your overall health risks.
  • How do I figure out my ideal healthy weight? Being healthy is about more than just a number on the scale. Talk to your doctor to determine your recommended weight range while considering other factors that influence overall health.

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Source: verywellhealth