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What is the BMI Formula?

Obesity is a growing problem in the United States. Being overweight is linked to a host of health problems including, cardiovascular disease, diabetes, hypertension and arthritis. Body mass index--BMI--is a measurement of the relationship between a person's height and weight, which also correlates to body fat. Knowing your BMI is an important factor in determining whether you are in a healthy weight range or not.

  1. Categories of BMI

    • There are four categories in BMI measurement. The term "underweight" applies to individuals with a BMI less that 18.5. "Normal weight" is assigned to those who score 18.5 to 24.9. A score of 25 to 29.9 is defined as "overweight," and "obese" is considered a score of 30 and above.

    BMI Formula Step 1

    • Multiply weight in pounds by the number 703. For example, a 145 lb. person would compute the following: 145 x 703 = 101,935.
      When using the metric system, the first step would be determining weight in kilograms. If you need to convert pounds to kilograms, use the following formula: Weight in pounds / 2.2 = Weight in kilograms. Therefore, for this example, 145 / 2.2 = 65.91 kilograms.

    BMI Formula Step 2

    • Square height in inches. That is, multiply height in inches times itself. For instance, a five-foot-five-inch person would be 65 inches tall. 65 x 65 = 4225.
      Step two using metric equivalents would be squaring height in meters. To convert inches to meters, multiply inches x .0254 In this case, multiply 65 (inches) x .0254 = 1.651 centimeters. Then square the answer: 1.651 x 1.651 = 2.7258

    BMI Formula Step 3

    • Divide the answer in step one by the answer in step three. The equation for this particular example would look like this:
      101,935 / 4225 = 24.1266272189... or to round the answer, 24.13.
      For metric purposes, step three is as follows: BMI = weight in kilograms / height in meters². For this example, compute the following: 65.91 / 2.7258 = 24.18
      Both answers would fall into the "normal weight" range of body mass index.

    Considerations

    • Body mass index is a valuable measurement, but when it comes to determining health and body condition, other factors must be considered as well. For instance, a person with a great amount of muscle mass may have a higher BMI, thereby categorizing the individual as being overweight. This doesn't mean that the individual is unhealthy, though, because the extra mass is muscle and not fat. Doctors also consider other factors such as heredity and pre-existing medical conditions, in addition to BMI, in terms of assessing overall health. Additionally, women have higher percentages of body fat than men on average, and body fat tends to increase for men and women with age.

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