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What is Body Mass Index (BMI)?

Updated on April 17, 2013
BMI Chart.
BMI Chart. | Source

Body Mass Index (BMI), also known as the Quetelet Index of Obesity, is a formula used to determine the degree of obesity of an individual. The formula was created in the nineteenth century by Adolphe Quetelet and became internationally used to measure obesity in the 1980s. It is still widely used today despite being very inaccurate.

Calculating Your BMI

For U.S. customary units, BMI is calculated by dividing a person's weight (lb) by their height squared (in2) and multiplying by 703. For the metric system it is the same equation but without a multiplying factor: weight (kg) divided by height squared (m2).

Example (U.S. Units):

Height=5'-6" (66"), Weight=150 lbs.

(150/662)*703=24.21

Example (Metric Units):

Height=170cm (1.70 m), Weight=70 kg

(70/1.702)=24.22

A BMI calculator can also be found here.

Interpreting Your BMI

BMI
Classification
< 18.5
Underweight
18.5 - 24.9
Normal
25.0 - 29.9
Overweight
> 30.0
Obese
Source

Why BMI Should Not Be Used

  • It is outdated. The formula for calculating BMI was created over 200 years ago by a mathematician, not a physician. Quetelet created the equation to help the government catalog the general obesity of the public. It was never meant to be used as a medical resource.
  • It does not account for the ratio of muscle, fat, and bone content. BMI is calculated strictly from height and weight, therefore many physically fit people find themselves in the "overweight" category due to their high muscle percentage. Stronger (and therefore heavier) bones can also skew the measurement.
  • Body frame and build type are also not considered. A person with a large frame (wide hips and shoulders, large bones) may be classified as obese when in reality they are not. Likewise, a petite person with a small frame could incorrectly be labeled underweight.
  • An inaccurate BMI can lead to medical mistreatment. For example, to be admitted into an in-patient treatment facility for anorexia, a BMI of 17.5 or lower is typically required. BMI is a made up number that is preventing suffering people from getting the help they need.

Alycia Kluegl on BMI

Alternatives

  • Calipers: Skin-fold calipers work by pinching the fat on your body and then reading the thickness. The measurement is given in millimeters and is then compared with a chart shower gender and age to find your body fat percentage. This method is generally very accurate, although if you are administering the test yourself it may take some practice.
  • Anthropometric: This method uses body circumference methods to calculate your body fat percentage. Typical measurements are waist, neck, hips, and height. The U.S. Navy uses this method and an online calculator can be found here. The upside to this test is that it is quick and easy, but it does have questionable accuracy.
  • Hydrostatic Weight: One of, if not the most, accurate method to finding body fat percentage. The test is performed by submerging the individual in a tank and finding how much they weight underwater. Because muscle is more dense than fat, a fit person will weigh more in the water and will have a lower body fat percentage. This option does require a hospital visit and can be expensive.
  • Bioelectric Impedance Analysis: This is an easy, relatively inexpensive option. BIA scales even come built in with some regular scales. What BIA does is measure the conductivity of your body. Muscle has higher water content than fat and will conduct the flow of electricity better. BIA can be pretty accurate but how hydrated the individual is can throw off the results.

There are several other methods including Dual Energy X-Ray Absorptiometry and Total Body Electrical Conductivity. These types of test involve very expensive, high-tech, and hard to find machinery. Some of them use radiation which limits their use and others require more research before being commonly used.

Can BMI Ever Be Used?

Yes, of course. BMI can be used to get a very general idea of someone's body composition. It should never be used to diagnose any medical condition. If you are a serious athlete or have health issues then it is recommended that you used a more accurate method of finding your body fat percentage.

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    • Larry Fields profile image

      Larry Fields 4 years ago from Northern California

      Hi Victoria Anne,

      Good job! I especially liked your descriptions of the more accurate methods for determining body fat.