One of the most confusing things when first learning about physical fitness can be all of the equations, formulas and indexes that are commonly used. Some of these are of great use while others hang on only to remind us of the true lack of evidence based health education on a global level. In this, the first of what will be a series of articles covering the common methods of data gathering for health and fitness I will cover the most used and often misunderstood metric: Body Mass Index (BMI).

Body Mass Index is the index of a person’s height relative to their weight. This index is probably something most readers have heard of before as it has become a common method for measuring body fatness and obesity — but I wonder, how many of you can explain to others how to calculate BMI without a Body Mass Index calculator? By the end of this article if nothing else, every reader should be able to do that so let’s get to it!

Calculating BMI

To calculate BMI use the following equations:

BMI = weight (kg)/height (m)² 1 2

or if using the imperial system:

BMI = [weight (lb)/height (inches)²] x 703 3 4 5

Note that some sources show a different conversion factor than the 703 used here 6. The conversion factor of 703 is the more accurate of the whole values out there as it results in a closer value to the one given if we were to convert pounds to kilograms and inches to meters (2.204623 lb/kg and 39.37001 in/m) individually rather than using the conversion factor. 703.07 is the most accurate if we wished to be exact but 703 is accurate enough for this type of measurement 7. Those who choose higher conversion factors probably did so due to rounding.

Now let’s take a look at these formulas in action using common values. The examples below are based on a 5’6 (66in or 167.640293cm which is 1.67640293m) person weighing 140lbs (63.5029209kg):

Metric:

First we are going to round to 1.676 as nobody measures their height to the micrometer or smaller, then continue.

1.676×1.676 = 2.808976

63.5 / 2.808976 = 22.6061027

If you are less precise and use 1.67 you will get 22.7688336.

Imperial

66 x 66 = 4356

140 / 4356 = 0.03213957759

0.03213957759 x 703 = 22.594123

To make it easier most (including calculators 8 9) round to the nearest tenth for BMI so in this case we get 22.6 as our value.

Result! How do we classify it?

It is not hard to see why BMI caught on — the result of this equation is a simple number and the value is easy to calculate, but how should the result be interpreted? Let’s explore further.The World Health Organization (WHO) currently defines Body Mass Index categories as the following:

Classification BMI
Underweight <18.50
Severe underweight <16
Moderate underweight 16·0-16.9
Mild underweight 17·0-18.49
Normal range 18.50-24.9
Overweight: >25
Preobese 25.00-29.9
Obese: >30
Obese class l 30.00-34.9
Obese class ll 35.00-39.9
Obese class lll >40.00

10 11

Although the above chart is the international classification for BMI, Asian populations have been identified as being at an increased risk of type 2 diabetes and cardiovascular disease at BMI values lower than the existing WHO cut-off point for the overweight category. The WHO expert consultation on the subject recommended adding the following additional cut-off points for Asian populations: 23, 27·5, 32·5, and 37·5 kg/m².  As such, Asian populations would have additional categories of 18·5–22.9 kg/m² increasing but acceptable risk; 23–27·4 kg/m² increased risk; and 27·5 kg/m² high risk. 32·5, and 37·5 kg/m² would be used to further classify obesity.

Where possible, in populations with a predisposition to central (abdominal) obesity and related increased risk of developing the metabolic syndrome, waist circumference should also be used to refine action levels on the basis of BMI.

BMI Prime

BMI Prime is the ratio of a person’s actual weight (or mass) to his/her upper weight (or mass) limit, it is also the ratio of a person’s actual BMI to upper limit BMI. In other words, BMI Prime = BMI value / 25 for non-Asian populations and BMI value /23 for Asian populations.  The following values correspond with the standard BMI classifications: less than 0.74 (underweight); between 0.74 and 1.00 (normal); 1.00 or greater (overweight). 12

Exceptions to the Rule

Body Mass Index as outlined here was originally determined to only be accurate for populations aged 20 and older.1 BMI is also used to assess weight in children, but through comparison to population standards for sex and age. BMI changes with age in healthy children. A BMI <5th percentile is underweight, a healthy weight is equivalent to a BMI between the 5th and 85th percentiles, BMIs between the 85th and 95th percentiles are classified as overweight, and a BMI > 95th percentile is considered obese for ages 2-20. Body weight and recumbent length for boys and girls under 2 years of age are assessed using growth charts.  These charts provide percentiles of weight for recumbent length.13

Issues with BMI

I’m sure many readers have noticed by now that populations are being referenced rather than individuals. This is where the issues with BMI as it is often used today come in. The originator of this index did not mean for it to be a measure of an individual’s health or obesity. The theory that weight increased in proportion to the square of height was originally thought of by a founder of the social sciences,  Adolphe Quetelet as what was originally known as the Quetelet Index. During a quest to better define what he referred to as “the normal man” he compared various populations to other populations in the world and to each other. He did not specify what range people should fit into for health.1 Those ranges came much later, defined by scientists decades after the initial publication of Quetelet’s work as they began to discover associations between BMI and health issues using population averages. Many issues exist with trying to apply BMI as a diagnostic method (meaning BMI at the individual level) but most concerning is ecological fallacy. Ecological fallacy is inference about an individual based on aggregate data for a group. 14 In reality, just because group data shows a correlation does not mean the same will hold true for an individual yet that is what BMI cut-off points are based on. 15 When thinking about Body Mass Index keep in mind that even Ancel Keys the man who coined the name Body Mass Index proposed that BMI was the best proxy for body fat percentage among ratios of weight and height believed it to only be accurate for populations not individuals.16

When we start to dig into the data relating to BMI’s accuracy and the implication of being overweight by BMI standards multiple concerns arise. A study published in 2012 found that BMI underestimates obesity prevalence, especially in women with high leptin levels (.30 ng/mL).17 Also in 2012 a study was published in Archives of Surgery showed better survival rates for those on the high side of normal range into overweight when compared to lighter individuals.  18

Conclusion

Most issues with the Body Mass Index come down to the fact that BMI does not measure tissue types in the way a more accurate test such as dual-energy X-ray absorptiometry (DEXA) does. Instead, BMI treats all tissues as equal. As an additional issue the medical community have used BMI in a manner which it was never meant to be used. This misuse confuses the general public and causes propagation of inaccurate information about healthy weight and body composition. BMI has a place in measuring population thickness and thinness as a whole but its use should be avoided when assessing the body composition and health risks of individual people.

  1. Quetelet, A. (1968). Chapter II. In A treatise on man and the development of his faculties,(pp. 63-67). New York: B. Franklin.
  2. Billewicz, W. Z., Kemsley, W. F. F., & Thomson, A. M. (1962). Indices of adiposity. British journal of preventive & social medicine, 16(4), 183-188.
  3. Thompson, J., & Manore, M. (2011). Achieving and maintaining a healthful body weight. In The science of nutrition (2nd ed., p. 487). San Francisco: Benjamin Cummings.
  4. Hark, L. (2014). Fundamentals of nutrition assessment. In Medical nutrition & disease: a case-based approach (Fifth ed., p. 83). Wiley-Blackwell.
  5.  Rosenthal, M., & Glew, R. (2009). Triacylglycerol transport and metabolism. In Medical biochemistry: human metabolism in health and disease (p. 188). Hoboken, NJ: John Wiley & Sons.
  6. Marieb, E., & Hoehn, K. (2013). Nutrition, metabolism and temperature regulation. In Human anatomy & physiology (9th ed., p. 939). Boston: Pearson.
  7. Palma, J., & Pittard, J. (2002, May 12). Body Mass Index and Uremia. Retrieved February 4, 2015.
  8. Calculate Your Body Mass Index. (n.d.). Retrieved February 8, 2015, from http://www.nhlbi.nih.gov/health/educational/lose_wt/BMI/bmicalc.htm
  9. Dietitians of Canada – BMI for Adults. (n.d.). Retrieved February 8, 2015, from http://www.dietitians.ca/Your-Health/Assess-Yourself/Assess-Your-BMI/BMI-Adult.aspx
  10.  Obesity: Preventing and managing the global epidemic: Report of a WHO consultation. WHO Technical Report Series #894. (2000, January 1). Retrieved February 8, 2015, from http://whqlibdoc.who.int/trs/WHO_TRS_894_(part1).pdf
  11.  Appropriate Body-mass Index For Asian Populations And Its Implications For Policy And Intervention Strategies. (n.d.). The Lancet, 157-163.
  12.  Gadzik, J. (2006). ” How much should I weigh?”–Quetelet’s equation, upper weight limits, and BMI prime. Connecticut medicine, 70(2), 81-88.
  13. Gropper, S., & Smith, J. (2012). Body composition, energy expenditure, and energy Balance. In Advanced nutrition and human metabolism (Sixth ed., p. 267).
  14.  Boslaugh, S. (2008). Encyclopedia of epidemiology. Thousand Oaks Calif.: Sage Publications.
  15. Nicholls, S. (2013). Standards and classification: A perspective on the ‘obesity epidemic’.Social Science & Medicine, 9-15.
  16.  Keys, A., Fidanza, F., Karvonen, M., Kimura, N., & Taylor, H. (1972). Indices Of Relative Weight And Obesity. Journal of Chronic Diseases, 25(6-7), 329-343.
  17.  Shah, N., Braverman, E., & Nizami, Q. (2012). Measuring Adiposity in Patients: The Utility of Body Mass Index (BMI), Percent Body Fat, and Leptin. PLoS ONE, E33308-E33308.
  18.  Turrentine, F. (2012). The Relationship Between Body Mass Index and 30-Day Mortality Risk, by Principal Surgical Procedure. Archives of Surgery, 236-236.