What the Body Mass Index Measures

Updated May 23, 2018
Bathroom scales on tiled floor
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BMI or Body Mass Index is a method of estimating a person's body fat levels based on a person's weight and height measurement. While the BMI calculation is an indirect measurement, it has been found to be a fairly reliable indicator of body fat measures in most people. Research on BMI calculations finds a strong correlation with other, more complicated direct measurements of body fat levels, including underwater weighing.

Because BMI is a simple calculation, anyone can use it to determine health risk due to excess body fat levels.

Other Body Fat Measurement Methods

There are several different methods of assessing the percent of fat and lean mass of an individual. These methods are referred to as body composition analysis. Some of the most common measurements include skinfold thickness, underwater weighing, and bioelectrical impedance.

  • Underwater weighing or hydrodensitometry is complex and complicated, so most experts use simple skinfold thickness measurements to determine body fat percent.
  • Bioelectrical impedance is another common method of assessing body fat percentage. This method determines total body weight, the percent and amount of body fat, muscle mass, water, and even bone mass. While the readings can be affected by hydration level and other factors, they provide fairly accurate readings over time. Body fat scales are available for home use that employ this method of measurement.

    BMI Formulas

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

    Example: Weight = 68 kg, Height = 165 cm (1.65 m)
    Calculation: 68 / (1.65) 2 = 24.98

    English BMI Formula: weight (lb) / [height (in)]2 x 703

    Example: If your weight is 150 lbs and height is 5’5” (65")
    BMI Calculation: [150 / (65) 2] x 703 = 24.96

    What Your Result Means

    You can interpret your BMI result using this simple chart. For adults, BMI results are interpreted as follows:

    • BMI below 18.5 = Underweight
    • BMI 18.5 – 24.9 = Normal weight
    • BMI 25.0 – 29.9 = Overweight
    • BMI 30.0 and Above = Obese

    Note that BMI is interpreted differently in children. Growth charts and percentiles are used. If they are at or above the 95th percentile of children their age they are considered obese.

    Limitations to BMI as a Body Fat Measurement Tool

    Even though there is a fairly strong correlation between the BMI and body fat measurement, there are some limitations based on an individual's gender, age, and athletic ability. These limitations include the following:

    • Women tend to have more body fat than men.
    • Older people tend to have more body fat than younger adults.
    • Highly trained athletes often have a high BMI due to higher levels of muscle mass which increases their body weight measurement, rather than higher body fat causing a higher weight measurement.

    Body Composition, Body Fat, and BMI

    Athletes who have higher levels of muscle mass need to be somewhat leery of the BMI calculation. Because the BMI number cannot distinguish the different components that make up total body weight, an athlete is better served by using a direct measurement of body composition and body fat than using a simple formula.

    Keep in mind that the BMI calculation is used to screen the general population for health risks related to having too much body fat. This is not a tool that works well for most athletes who are curious about their own body composition that includes lean mass and fat mass.

    Health Risk Related to a High BMI

    The reason the BMI is used for screening the health of the general population is due to the strong correlation between being overweight or obese and having health problems, chronic disease, and premature death. People who are overweight or obese have an increased risk for the following health conditions:

    Source:

    CDC Health Information, About BMI for Adults, Centers for Disease Control, 2008.

    Prentice AM and Jebb SA. Beyond Body Mass Index. Obesity Reviews. 2001 August; 2(3): 141–7.

    Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults, National Institute of Diabetes and Digestive and Kidney Diseases, 2008.