Brian Greenlee by Brian Greenlee
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A frequently used index to categorize a person's body weight is called the body mass index, or BMI. This assessment compares your body weight to your height to come up with a value that indicates whether you are underweight, normal weight, overweight, or obese. Although BMI is often used as an indicator of body composition (i.e., the ratio of fat mass to lean mass), it does not take into account body fat and can only be used as a method to evaluate body weight.

The following formula is used to calculate Body Mass Index:

BMI = Weight (kg) / (Height (m)²)

 

Practical example:

Find the BMI of an individual who is 5'7'' tall and weighs 150 pounds.

Weight conversion (lb to kg): Weight in pounds divided by 2.2
Example: 150 lb ÷ 2.2 = 68 kg

Height conversion (inches to meters): (height in inches x 2.54) ÷ 100
Example: 67” x 2.54 = 170
170 ÷ 100 = 1.70 meters

BMI = 68 ÷ (1.7)² = 68 ÷ 2.89 = 23.5

Knowing a client's BMI can give you a quick indication of his or her risk for weight-related health concerns. For example, those with a BMI >30 are classified as obese, which can lead to a number of diseases such heart disease, type 2 diabetes, hypertension, and stroke. BMI can also be used in determining a client’s cardiovascular risk classification. When completing a cardiovascular disease (CVD) risk factor assessment, an individual who has a BMI >30 has a positive risk factor associated with CVD. This can put the individual at a higher risk for a cardiac event during moderate to vigorous exercise. For those with overweight or obesity, losing weight can have a profound positive effect on overall health.

The table below provides established BMI norms based on various categories of health and fitness.

BMI Reference Chart

Weight Category

BMI Range

Underweight

<18.5

Normal Weight

18.5-24.9

Overweight

25.0-29.9

Grade I Obesity

30.0-34.9

Grade II Obesity

35.0-39.9

Grade III Obesity

>40

*Source: ACE Health Coach Manual page 300 table 11-5 

For most people, BMI is a quick and easy way to categorize body weight, which is why it’s so commonly used. However, since body weight—not percent body fat--is used in determining BMI, it may not be the best way to assess those who are heavily muscled (such as body builders) or who have an athletic body type. Because muscle weighs more than fat, those who have well-developed muscles typically appear overweight or obese according to the BMI reference chart. We do not recommend using BMI as the only method to categorize body weight or health risk for those with an athletic of muscular build. An estimation of body fat using the skinfold method would be a more accurate choice.