What is body mass index (BMI)?

Diana Meeks
Diana Meeks on behalf of Sigma Nursing
Family Practitioner

The body mass index (BMI) is a special measurement used to determine whether your weight is healthy for your height. There are five classifications of BMI: Underweight (below 18.5), healthy (18.5-24.9), overweight (25.0-29.9), obese (30.0-39.9) and morbidly obese (more than 40). Knowing your BMI is important, because being overweight puts strain on your heart and can lead to serious health problems.

BMI stands for body mass Index and is an index of a person's weight in relation to their height. BMI is a common technique used to determine thinness and excessive fatness in the body. Because of its simplicity and consistency, BMI is the most commonly used method in determining overweight and obesity in the general population. It is also commonly used for determining health risks associated with excessive body fat. Research shows that health risks start to increase when the BMI exceeds 25.0—which classifies an individual as being overweight.

Jonathan Penney
Fitness Specialist

Your BMI is your height to your weight. Body mass index is measured on a number scale and being between 18.5 and 24.9 is considered to be a normal weight range. Anything below 18.4 is considered underweight and anything above 25 is considered obese.

Body mass index, often referred to as BMI, is a measurement designed to assess prevalence of obesity-related diseases. It is important to note that BMI does not assess body fat, it is merely a method to determine whether or not an individual’s weight is appropriate to their height. Therefore, BMI may have a certain degree of error associated with it. For example, if a 200 pound 5’6” bodybuilder with a body fat of 6 percent calculates their BMI to be a little more than 32 they would appear to be obese. But, clearly the body builder is not battling obesity, the calculation just fails to account for the large amount of muscle. BMI is one of the most widely used measures because there is a minimal amount of user calculation error. Most people can measure height and weight accurately. To determine BMI, take weight (in kilograms) and divide by height (in meters squared). The result of this equation is the individual's BMI. There are three classification ranges of BMI which will determine the risk factor associated with obesity. Mild risk for obesity-related diseases occurs for those with a BMI between 25 and 30. Moderate risk for obesity-related diseases occurs for those with a BMI between 30 and 35. Sever risk for obesity-related diseases occurs for those with a BMI greater than 35.

Wendy Batts
Fitness Specialist

Body mass index (BMI) is a ratio of someone’s height and weight. It is one way to measure body fatness and is widely used because it is easy to perform, inexpensive and doesn’t require any tools and processes other than a calculator and a formula. While it doesn’t directly measure body composition or body fat, it correlates well to the actual fat content of the body and is a reliable measure.

It is used primarily to place someone in a weight category and serve as an additional tool in assessing their risk for chronic diseases related to being overweight and obese.

Mrs. Marjorie Nolan Cohn
Nutrition & Dietetics Specialist

Overweight and obesity are both labels for ranges of weight that are greater than what is generally considered healthy for a person’s height. The terms overweight and obese also identify ranges of weight that have been shown to increase the likelihood of certain diseases and other health problems.

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.

It is important to remember that although BMI correlates with the amount of body fat, BMI does not directly measure body fat. As a result, some people, such as athletes, may have a BMI that identifies them as overweight even though they do not have excess body fat.

Dr. Mike Clark, DPT
Fitness Specialist

The BMI (body mass index) is a mathematical formula that factors a person’s height and weight in determining obesity. It may be less accurate for athletes or older persons who have lost muscle mass.

BMI, what does it mean?
Weight               BMI
Underweight      Below 18.5
Normal               18.5 - 24.9
Overweight        25.0 - 29.9
Obesity              30.0 and Above

You can use the tool below to calculate your BMI:

Body mass index (BMI) is a number calculated from the relationship between a person's weight and height. A BMI number or "score" places individuals in weight categories. These categories can range from severely underweight to morbidly obese. Having a low BMI can place someone at risk for the following health conditions and diseases; osteoporosis, impaired immune system function, amenorrhea, cardiac abnormalities and more. Obesity and morbid obesity, or having a high BMI score are risk factors for blood (fat) lipid abnormalities, cancer, including cancer of the uterus, cervix, ovaries, breast, colon, rectum and prostate, depression, gallbladder disease, gynecological problems, such as infertility and irregular periods, heart disease, high blood pressure, metabolic syndrome, nonalcoholic fatty liver disease, osteoarthritis, skin problems, such as intertrigo and impaired wound healing, sleep apnea, stroke, type 2 diabetes according to the Mayo Clinic.
Below 18.5 = Underweight
18.5 – 24.9 = Normal
25.0 – 29.9 = Overweight
30.0 and Above = Obese

Body mass index is a mathematical formula that helps determine whether people are at a healthy weight. The calculation of BMI takes into consideration height and body weight to determine whether a person is at a healthy weight, or is underweight or overweight.

For example, a person who's 6'4" and weighs 200 pounds is probably at a healthy weight. But someone who's 5'9" and weighs 200 pounds is probably overweight.

Broadly speaking, the BMI helps officials get a general idea of how weight and obesity affect the population's health. With individuals, it enables a doctor to identify weight problems in a patient before serious health conditions arise.

Studies show that being overweight or obese can greatly increase a person's risk for developing conditions such as heart disease, diabetes, osteoarthritis or some types of cancer.

And being underweight can also lead to greater health risks from malnutrition.

Body mass index (BMI) is a measure of your weight in relation to your height. BMI is an assessment tool for determining whether your weight falls within one of four categories.

People with BMIs under 18.5 are considered underweight. Your weight is said to be within the healthy range if your BMI is 18.5 to 24.9. People with BMIs between 25 to 29.5 are considered overweight, while those whose BMI are above 30 are obese.

Remember, one size does not fit all. Other factors such as body build, age and ethnicity are not considered in this equation.

Body mass index (BMI) measures the relationship (or ratio) of your weight to your height. It is an estimation of body composition based on the notion that your body weight should be proportional to your height.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.