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What Is BMI?

What is BMI and is it an Accurate Measure of Weight and Health?

Dear Fitness Nerd,

Can you explain exactly what BMI is? I recently bought a Wii Fit and according to the BMI calculation Im fat! I checkedWhat is BMI some other BMI calculators online and am getting similar results. But Ive never really considered myself overweight. I exercise daily and think I look pretty toned, but according to the BMI charts Im overweight! Should I be worried? Erika (Atlanta, GA)

Ericka, the Body Mass Index, or BMI is one of those things that I wish would just go away, since it causes all kinds of anxiety for people who may actually be in very good shape. It also makes people who may actually have high body fat levels focus on the wrong thing: losing scale weight, when they should be focusing on reducing body fat and encouraging muscle building.

While BMI can be a tool for gauging a persons body composition in relation to height and weight, it tells you nothing about a persons percentage of body fat. So for people who have low body fat levels with substantial amounts of lean tissue (muscle), it can be very misleading.

But before we get into this in greater detail, lets take a look at what BMI is.


History of BMI: Who Invented the Body Mass Index and Why Is It Used?

The modern BMI or body mass index formula is based on an algorithm invented in the mid-19th Century byAdolphe Quetelet, a Belgium statistician, astronomer, mathematician and sociologist. While some adjustments have been made to Quetelets early formula over the years, the BMI calculation at its core is just a comparison between your height and weight. Technically, its your body weight divided by the square of your height.

BMI started to become a popular front-line diagnostic tool for obesity in the 50s and 60s.  The idea was that doctors and nutritionists could use BMI to more objectively evaluate a persons weight, since the term overweight or fat can be be subjective. By assigning a numerical value to fatness or thiness and comparing that to a broader population, BMI was intended as a starting point for diagnosing obesity, not as the final say.

Because BMI can be calculated using two pieces of information nearly everyone knows about themselves their height and weight BMI has become ubiquitous, cropping up in everything from the Wii Fit, to online weight loss websites and fitness tracking tools, to actuarial tables for health and life insurance.  The World Health Organization (WHO) even uses a modified version of BMI to measure and track obesity or malnutrition in global populations.

There are actually two different BMI formuals out there: one for adults and a separate BMI calculation for children and teenagers.

While BMI is still in use by some personal trainers, exercise physiologists and nutritionists many health and fitness professionals have moved away from BMI in recent years, and have started to embrace more accurate measures of body composition, like body fat percentage.

How To Calculate BMI

Calculating BMI is actually fairly simple, which accounts for its popularity.

Body Mass Index is a statistical formula that basically takes your height and weight and calculates a factor or body mass number that is then cross referenced against a index that places you in one of four ranges:

  • Underweight
  • Normal
  • Overweight
  • Obese

So all you need to calculate BMI is your height, weight and a standard BMI chart like the one below (courtesy of the U.S. Surgeon General.)


You simply take your height and current weight and then find the nexus point on the BMI chart and you have your BMI. The chart will then show you which BMI weight band you fall into. While there are BMI calculators available online, the process is so easy, that they really are unnecessary (provided you have the BMI chart handy.)

Problems with BMI as an Accurate Measure of Body Composition

While BMI continues to be a popular way to get an initial sense of whether a person has a healthy body weight, there are a number recognized issues with BMI:

  • Its based on assumptions that may not apply to all people
  • It doesnt take into account body fat percentage
  • It doesnt distinguish differences in genders
  • It tends to under-estimate body fat in the elderly
  • It doesnt tell you much about a persons individual body composition
  • It over-emphasizes scale weight as a measure of ideal body composition
  • It doesnt account for other life-style risk factors aside from weight

Lets take a closer look at these specific issues with BMI.

BMI: Watch Out for The Assumptions!

The first problem with BMI is that Body Mass Index is based on some assumptions that wont necessarily apply to everyone.

The standard BMI calculation assumes that the individual being evaluated is sedentary with average body composition. So BMI will often be inaccurate out of the gate for athletes or other individuals who carry a substantial amount of lean mass as a ratio of their overall body weight.

An Example of Why BMI Can Be Deceptive

To illustrate the problem with BMI, take two men of equal height and weight. One man is 5 11 tall at 180 lbs with 13% body fat, while the other man is the same height and weight, but has 23% body fat. BMI in both cases will indicate that the subjects are overweight.

However, is this really true?

The man with 13% body fat has 156 lbs of lean mass, and 24 lbs of fat. The other man, at the same height and weight, has 138 lbs of lean mass and 42 lbs of fat. In other words, hes almost twice as fat as the first example, even though their BMI shows them as equally overweight. The male with 13% body fat would actually be considered athletically lean based on standard body fat percentage charts, while the male with 23% body fat would be borderline obese.

So while BMI was more or less correct for the person who carried a higher proportion of body fat, it was extremely inaccurate for the athletically-lean person. The reason this happened is because BMI assumes that both individuals have similar body compositions and activity levels, when clearly, they dont.  In fact, the more lean mass you put on, the less likely it is that BMI will accurately reflect your ideal weight.

BMI Doesnt Account for Age or Gender

BMI also doesnt take into account gender or age (although there are modified version of BMI that try to correct this), so it may over-estimate body fat in younger people, athletes and women, while under-estimating body fat in the elderly (this is most likely explained by increased age-related loss of lean tissue as well as loss of height with age.)

Women tend to naturally carry more body fat then men, but BMI fails to take this into account. It treats both genders equally. In fact, as I discuss below, some recent research has shown that BMI is a less accurate measure of actual body fat percentage in women than it is in men.

BMI Assumptions About Height

BMI will typically show taller people as leaner, while shorter people as overweight or obese regardless of their actual body fat percentage.

While taller people do tend to have slimmer builds than shorter people, BMI deals purely in the averages, not the specifics.  There are obese 6 5 men, just as there are very lean 5 5 men.  So when you are dealing with BMI, its important to always remember that BMI makes generalizations, and is not specific to individuals. The only way to truly measure an individuals specific body composition is through something like body fat percentage analysis.

Is BMI Counterproductive and Misleading?

As we already discussed, BMI also doesnt tell you much about a persons specific body composition in particular, the proportion of body fat, muscle, bone, cartilage and water water.

These aspects of body composition are important when designing an individualized fitness plan, because a persons goal should generally be to reduce body fat percentage, not merely scale weight.

An over-emphasis on scale weight reduction can actually be counterproductive to increasing overall physical fitness, because in the process of losing weight, a person can lose more muscle than fat especially with diets that utilize extreme calorie-restriction to force loss of scale weight. In these cases, a persons body fat percentage can actually increase, even though their scale weight is decreasing. So even though you are losing weight, youre actually becoming fatter.

If you use body fat percentage as a guide for improving body composition, you can typically avoid this effect, since body fat percentage is a calculation of your total body fat in relation to your weight and lean tissue.

Is BMI Ever Accurate? What The Research Says

A 2008 study from Romero-Corral et alpublished in the February 2008 edition of the International Journal of Obesity seems to confirm some of the accuracy problem with BMI. That study took a look at 13,601 subjects (a very large sample, by the way) and compared their BMI to body fat percentage using bioelectrical impedance analysis.

The researchers found that 19.1% of the men and 24.7% of the women in the sample were classified as obese using BMI, while body fat percentage analysis showed that obesity was present in 43.9% of male subjects, and 52% of female subjects. So in this case, BMI actually under-diagnosed obesity in both groups. In fact, in men overall, BMI was more accurate than body fat percentage in measuring obesity, and in women, it was less accurate.

This may have to do with the changing body composition of individuals over the past several decades, as inactivity and reliance on convenience food has shifted the average body composition of people toward a higher percentage of fat versus lean tissue.

However, before you take this to mean that BMI is actually an accurate gauge of body composition, its important to note that in the intermediate BMI range (25-29.9 kg), BMI failed to discriminate between body fat percentage in both males and females. So for people that are in the middle of the BMI range, the correlation between BMI and actual body fat percentage didnt hold. BMI also had a weak correlation to body fat percentage in the elderly.

The takeaway: BMI seems to be fairly accurate on the two extremes of the scale: people who are very light and people who are very heavy for their height. However, within the middle range, its accuracy starts to break down.

Does a Lower BMI Mean Youre More Healthy?

Finally, its important to understand that a lower BMI does not necessarily directly correlate with increased health or risk for disease.

Again, BMI is simply a formula based on your height and weight and some assumptions about averages in the population. You can have a very low BMI and be borderline anorexic, for example, which can cause all kinds of short and long-term health problems.

You also could have an acceptable BMI and smoke, drink and eat fatty foods and have greater risk for developing cardiovascular disease, diabetes and cancer than another person with a higher BMI.

As always, the best way to measure physical fitness and health is holistically by looking at a persons diet, level of activity, lean muscle mass and overall lifestyle. Simply looking at a number on a BMI chart will never tell the whole story.

Alternative Methods to BMI for Determining Body Composition

So if BMI has accuracy issues, what should a person be using to determine body composition and gauge improvements in physical fitness and leaness?

There are a number of alternative formulas to BMI floating around out there, including equations that use hip-to-waist ratios or tape measurements of chest, waist and hips to try to better estimate body composition. While these formulas may be slightly more accurate than BMI, the best method for actually determining body composition is through % body fat analysis.

Body Fat Calipers: The Best Method of Determining Body Composition

There are number of tools available to measure % body fat, but the most accurate (compared to cost and convenience of more sophisticated methods like hydrostatic weighing, MRI, or Dual X-ray Absorptiometry) are body fat calipers.

Body fat calipers allow you to measure body fat by taking either single or multiple skin fold measurements across your body. You then use the measurements to calculate your % body fat.

Accu-Measure makes an inexpensive pair which the company claims are accurate within 1.1% of hydrostatic or under-water weighing, when used properly. Independent evaluations have confirmed this as well.  The best part is these calipers can be used effectively with only one skin-fold measurement.

Using Body Fat Percentage to Track Fitness Progress

The important thing to remember is that %body fat is most effectively used as a gauge for tracking your progress when attempting to decrease body fat, and not necessarily as an absolute method for determining leanness compared against an average population.

So even if your body fat measurements put you in an overweight category, you shouldnt let this discourage you. Your goal should be to reduce that body fat percentage over time, not just to compare yourself to other people.

Using %body fat also helps you avoid the trap of using scale weight alone as the gauge of your progress, which is counter-productive for many people and causes unnecessary anxiety especially if they add lean muscle in the process of increasing exercise and activity.

Utilizing %body fat will allow you to focus on increasing your lean body mass (LBM) percentage, versus simply reducing scale weight, which has overall health and metabolic advantages. It can also alert you when you may be losing more lean body mass than you are body fat, which can result in a fitness death spiral. If this happens, youll want to nip it in the bud right away with modifications to your diet.

What About the Wii Fit BMI?

In terms of your specific BMI on the Wii Fit saying your are overweight, its really difficult to say whether this is accurate or not, without knowing more about your body composition specifically your % body fat or LBM.

If you are athletic and are carrying a fair amount of muscle on your frame, there is a good chance that your BMI may be misrepresenting your true % body fat and body composition.

If you want to be certain, the best idea is to pick up some calipers and do a gut check (no pun intended) on the Wii Fits BMI calculation.  You can typically pick up a pair for under $10 at sporting goods stores, gyms or online.

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Comments (5)

  1. ronslavick (1 comments) says:

    Thanks for providing the valuable information about BMI.

  2. AnnetteM (1 comments) says:

    Thank you! Awesome article, and I will check out those body-fat calipers.

  3. Michael Howell (1 comments) says:

    RE: BMI inaccuracies.  I would imagine that none of the alternative calculators consider the body trunk length to leg length in making their calculations.  I am 59 and have a 29 inseam (short legs for my height).  This surely affects BMI.

  4. brandon morris (1 comments) says:

    thatnxs for the info peeps

  5. Jennifer Johnson (1 comments) says:

    My father is too much wide awake about ma health and weight. He keeps on checking BMI on monthly basics. It always tells that I am a lil over weight I tried to maintain my physique and checked again but still it was the same. i never felt that I am plump or overweight. thanks at least I can show him this article and make him satisfy about my health and the fault about BMI, the example for example of two men with the same height and weight is the right way to illustrate it.