What Is BMI? And Why Do I Care?

Live Fit | BMI Chart. What is BMIThis article will define BMI and other commonly used acronyms in the fitness industry and give a biased review of BMI.

What is BMI?

BMI stands for Body Mass Index. It is calculated using a person’s height and weight. BMI is the latest of a version of height and weight charts that were originally created to quickly and easily categorize a person’s physique. BMI is measured with the following metric formula: Weight in kilograms divided by height in meters squared, or kg/m2.


Live Fit | Dudley Alan Sargent

Dudley Alan Sargent

Beginning in the late 1870s Dudley Alan Sargent measured nearly every student that was enrolled in Harvard University physical education classes. He amassed a tremendous amount of data in the decades he was there. He gathered biometrics on student’s strength, flexibility, posture, phrenology, body circumference and, of course, height and weight. From the latter he created a chart that  would classify a person. The result would indicate whether the person was overweight, under weight, or normal.

Live Fit | Hemenway Gymnasium

Hemenway Gymnasium was directed by Sargent for several decades.

Insurance Companies

These height/weight charts were soon used by life insurance companies as a means to justify their rates to customers. They knew that too much or too little body fat was not healthy, but it wasn’t until Sargent made this analysis public that life insurance companies had a scientific basis and justification for this assumption. It wasn’t long before others would use Sargent’s charts for their purposes. Since that time height/weight charts have been used by insurance companies, the medical industry, research, statisticians, and the government.


Have you ever wondered how statistics such as “63% of the US population is overweight” are known? BMI is how. Have you ever taken a survey that asked for your height and weight? If so, you are part of the statistics. Are these statistics accurate? Somewhat.


Live Fit | same body weight - different body compositionThe problem with BMI and other height/weight charts is that they do not take into account different body types. A “normal” BMI range for a person is between 18.5 and 24.9. If you fall above or below that range you are considered over or under weight. The problem is, when over (24.9) weight, it is assumed that you are carrying too much body fat. However, this is not always the case. I, for example, hover just below the dreaded BMI 25 mark. If I were to gain 5 pounds of fat, muscle, water, or hair, I would be overweight. Period. And honestly,even if I were to gain five pounds of additional body fat would not put me in a category of excess body fat.

Then Why is My BMI So High?

Live Fit | Fat vs. Muscle

Pound for pound, muscle is half the size of fat

Because I am disproportionate from the “norm”. BMI, and weight, for that matter, does not take into account the components of a person’s body. A person’s body weight is made of many different components: skin, bone, muscle, water, hair, food in the gut, etc. and stored fat. Fitness professionals normally use body composition as an indicator of a person’s fitness and health. This composition is broken into two compartments – lean body mass and fat mass. As mentioned before, lean body mass is everything but fat. Many components can change a person’s body weight, but only fat and muscle are important when discussing health and fitness (not to drift off the subject with other factors like water and minerals).

What IS Important?

The ubiquitous “WEIGHT” that is so easily discussed, measured and fretted over, IS NOT IMPORTANT. Body composition is. The percentage of body fat compared to total body weight is what determines whether a person is too fat or not.  That bares repeating. The percentage of body fat compared to total body weight is what determines whether a person is too fat or not. A person can be at their ideal weight according to height, but over fat when measured with an accurate body fat percentage device. Conversely, a person can be “overweight” according to their height and be lean.

How Much Fat is Too Much?

These numbers vary depending on who is publishing the information, but from my years of studying the subject I have narrowed it down to a body fat percentage of 12% – 17% for men and 17% – 23% for women. The latter is most debatable because women’s bodies are the most unique and many can carry more fat then 23% and they don’t look or feel over fat. Women need more body fat than men. I have seen healthy body fat for women listed as high as 35%. So, consider the source when you hear this information. Some times an organization will adjust a “norm” based on compassion rather than healthy physiology. For example, there is no healthy reason a person’s body fat should increase (or decrease) outside of the healthy range as they age. If you see an ideal body fat listing based on age, this is for compassionate reasons, not health.

But Why All This Attention to BMI?

It is easy. BMI is a pretty common number that is easily attainable and purports to have value. It is nearly as common of knowledge as weight.  But, people used to think the world was flat.

There is confusion with a lot of the exercise physiology, health, and fitness terms that get thrown around: BMI, BF, BMR, RMR, BIA, LBM, HR, MHR, RHR, VO2max, BP, LDL, HDL, just to name a few. If you do not know what all these stand for and what they mean, you are not alone.

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Fitness Industry Acronyms

The first groups of acronyms are related to body fat in some way. The following is a list of these abbreviations and their meaning. (I’m not going to give extensive information on these as it would make the article too long and I don’t think it is pertinent at this point. If you’d like to know more, please ask me.)

  • BMI – Body Mass Index.
  • BF – Body Fat.
  • BMR – Basal Metabolic Rate; the amount of energy your body requires for basic life support, i.e., metabolism.
  • RMR – Resting Metabolic Rate; essentially the same as BMR.
  • BIA – Bioelectrical Impedance Analysis; An inaccurate, yet commonly used, means for measuring body fat.
  • LBM – Lean Body Mass; Everything except fat on a person’s body.

These other terms are common in my world and I’m occasionally asked about them.

  • HR – Heart Rate; in beats per minute.
  • MHR – Max Heart Rate; the fastest a person’s heart is able to beat.
  • RHR – Resting Heart Rate; the slowest is regularly beats in a resting state.
  • VO2max – Maximum Volume of Oxygen used. This is usually measured during exercise to give a performance level.
  • BP – Blood Pressure.
  • LDL – Low Density Lipoprotein; bad cholesterol.
  • HDL – High Density Lipoprotein; good cholesterol.


What is BMI? It is NOT a measurement of body fat; it is a measurement of body weight compared with body height. Body weight is comprised of many different things and only one of those things is fat. Understanding this difference will help you better understand what you can do about it if you want to change your condition and become more fit and healthy.

Live Fit | body circumference and skinfold

Measure body circumference and skinfold as an easy, accurate, and effective (and inexpensive) means for determining body fatness.

So, the next time someone talks to you about their BMI, you can inform them that it may not be very meaningful and suggest they have their body fat measured by a professional to determine their body composition (which IS a meaningful measurement). Once this number is established, it can be used as a benchmark for future measures to see how much muscle mass you are building (or losing) and if your efforts are changing the amount of fat you have stored on your body. BMI and/or weight will not do that.

If you have any questions about “What is BMI?” or any part of this article, please leave me a comment below. I may write an article on your suggestion.

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