BMI (Body Mass Index)

Body Mass Index (BMI) Overview

Body Mass Index (BMI) is determined using an individual’s height and weight. For most of the population, BMI can accurately assess body fat percentage. Body Mass Index is not a direct measurement of body fat, but studies indicate there is a direct relationship between BMI and direct measures of body fat, like underwater weighing and dual energy x-ray absorptiometry (DXA).1, 2 BMI is another way of expressing the measure of body fat.

BMI is a cheap and simple way to screen patients for further examination for potential weight related health issues.

What Does BMI Tell Us?

BMI is used to identify potential weight problems in adults. But the Body Mass Index is not a tool for diagnosing. A patient might have a high BMI, for example. A healthcare provider would have to conduct additional tests to determine if the excess weight actually creates a health risk. Also taken into consideration will be skinfold thickness measurements, diet analysis, exercise levels, family history, and other appropriate health screenings.

Is there a reason the CDC utilizes Body Mass Index to measure overweight and obesity?

Computing the BMI is the best way to determine the rate of obesity in the population. It is a cheap and efficient way for clinicians and for the general public to calculate BMI as it only requires height and weight. BMI allows individuals to see how their weight compares with the population at large.

Visit the section titled “How is BMI calculated and interpreted?” to view the calculation in either Metric or English units.

How else can obesity be measured? Why doesn’t the CDC use these other methods to screen for obesity in the general population?

You can also calculate body fat by measuring how thick the skinfold is using calipers, taking the weight underwater, using bioelectrical impedance, using dual-energy x-ray absorptiometry (DXA), and with isotope dilution. But these methods have drawbacks, such as availability, cost or need to have specialized staff. And different observers and machines can produce different results with many of these methods, which makes cross-study and cross-time period comparison difficult.

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What is the method for calculating and reading?

Children and adults both have their Body Mass Index calculated in the same way.

We calculate it like this: Measurement Units Formula and Calculation Kilograms and meters (or centimeters)

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

Using the metric system, we calculate BMI by dividing a person’s weight (in kg) by their height (in [m]2.) To calculate height in meters, take the height in centimeters and divide the number by 100.

For instance, for a person weighing 68 kg and 165cm (1.65m) tall the calculation is: 68 ÷ (1.65)2 = 24.98 To calculate using pounds and inches the formula is: weight (lb) / [height (in)]2 x 703. You determine BMI by taking a person’s weight and dividing it by their height inches squared, and then you multiply it by 703.

For instance: For a person weighing 150 lbs and standing 5’5″ (65″), the calculation is: [150 ÷ (65)2] x 703 = 24.96

Interpreting BMI for adults: For anyone over 20 years of age, you evaluate BMI utilizing common weight classifications that are equal for all ages and for both males and females. Teens and children, as a comparison, have interpretations of their BMI that are based on age and sex.

How well does the BMI represent the amount of fat on a body?

There is a strong correlation with fat and the BMI number, but there are differences that vary by age, race, and sex.

A woman who has the same BMI as a man often has more body fat. See examples 3 and 4 for variations.

Even though the BMI may be the same, an older person tends to have more body fat than a person who is younger.

A person who is very athletic may have a higher BMI because of extra muscle, not body fat.

Keep in mind that BMI is only one thing to consider when considering risk for disease.

If you want to forecast the chances that a person will become sick from being overweight, the National Heart, Lung, and Blood Institute suggests examining two other factors: How big the person’s waist is (excess abdominal fat tells us a person’s chances of developing obesity-related problems).

Other risk factors for obesity and related conditions, like high blood pressure or a sedentary lifestyle.

To learn more about forecasting one’s risk for becoming sick from being overweight, you can go online and see these pages hosted by the National Heart, Lung, and Blood Institute: Evaluating Your Risk, BMI Table, Clinical Guidelines on Identifying, Assessing, and Treating of Overweight and Obese Patients.

If someone is extremely active or athletic and has a BMI of more than 25, do we say they are carrying too much weight?

A person with a BMI over 25 is defined as overweight while anyone with a BMI over 30 would be defined as obese according to the BMI weight status categories.

It is important to note that BMI does not calculate the percentage of the body that is fat and that BMI is derived from a person’s total weight which includes both muscle and fat. Because of this some people have a high BMI but not a high amount of body fat. As an example, specialized athletes might have a high BMI because of the amount of muscles on their body, not fat. While some people are in the overweight range of i.e. a BMI of 25.0 to 29.9 – they may not have excess body fat while most people classified as obese i.e. a BMI over 30 – will almost certainly exhibit increased levels of body fat.

Note, though, that the risk of contracting disease depends on more factors than weight alone. You should discuss any questions or concerns you have about your weight with your doctor.

What medical complications may arise out of being overweight or obese?

The BMI scale is founded upon correlation between weight and illness and mortality.5 Overweight and obese people are more likely to develop certain illnesses, like: 6 High blood pressure, Dyslipidemia (high LDL cholesterol, low HDL cholesterol, or high levels of triglycerides), Adult-onset diabetes, Cardiovascular disease, Stroke, Gallstones, Osteoarthritis, Sleep apnea and breathing difficulties, and Certain cancers (endometrial, breast, and colon). To learn more about the medical aspects of overweight and obesity, you can read Clinical Guidelines on Identifying, Assessing, and Treating People Who Are Overweight.

Does one interpret BMIs for teenagers and kids in the same manner one does for adults?

Although the calculation of BMI is the same for adults and children, the interpretation of the results is done differently.

Because the percentage of body fat changes as one ages, the BMI sex specific- and age- percentiles are used for teenagers and kids. Girls and boys will have different body fat percentages as well.

Therefore, the meaning of the BMI is different depending on the age and gender for those under the age of 18. The BMI-for-age growth charts devised by the CDC account for this by allowing the BMI to be translated into an age and gender percentile.

BMI calculations for adults, however, do not take sex or age into consideration.