Why Do We Calculate BMI?

BMI is an acronym for “Body-Mass Index.” Your body-mass index is the measurement that is most often used to determine how overweight (or underweight) you might be. If a health care professional has ever told you that you were overweight, underweight, or obese, this is probably the method by which they arrived at that conclusion. It’s a relatively simple method that represents a good marker for keeping track of your overall health.

To find your BMI, you take your weight (in kilograms) and divide it by the square of your height (in meters). If you don’t want to do that much math, you can use this handy online BMI calculator, of which there are many. Anything below 18.5 is considered “underweight,” while anything over 24.9 is considered “overweight.” A BMI of 30 or more indicates obesity.

Benefits Of The BMI Method

The BMI method is popular for several reasons. First, it gives physicians and other health professionals a quick and easy way to evaluate a person’s weight.

This method also serves as an excellent fitness benchmark. While it may not always reflect a person’s true status, it does give them a rough idea of where they stand. For instance, your BMI measurement might say that you are “obese” when you are just “overweight.” This can happen due to limitations in the method (which we discuss below). Still, it gives you a way to track your weight-management efforts.

How Accurate Is The BMI Method?

There are a lot of mixed opinions when it comes to the BMI method. No one would deny that it has value, but many people find it to be a poor indicator of overall health. Instead of wading into that debate, let’s look for some evidence and see what it might show.

Here is a study that focused on the use of BMI in pediatric medicine. Researchers conducted a systematic review of the evidence, looking for any study that related to their question: Does BMI accurately predict a person’s actual levels of body fat? As far as pediatric medicine is concerned, the answer was mostly positive.

They admit that BMI does not always accurately predict the percentage of a child’s body weight. However, they maintain that it is crucial as a relative measurement. By comparing a child’s BMI to that of their peers, you can get an idea of how overweight or underweight they are. Due to the rising problem of childhood obesity, doctors consider it to be a very useful tool.

Here’s another interesting study. This one focused on the difference between different ethnicities in terms of BMI accuracy. Because people of different ethnicities tend to have slightly different body structures, you can’t necessarily use the same BMI scale for everyone. This study would seem to confirm that fact. They found the BMI method to be pretty inaccurate for Asian people in general, as well as Pacific Islanders. This study also confirms that BMI needs to be adjusted for ethnicity.

Here is another study, and this one focused on the general adult population. They used a method called bioelectric impedance to determine the body-fat percentage of all the test subjects. Then, they measured the height and weight of the participants and used those numbers to calculate their BMI value. The researchers found considerable differences in the results. Here is a summary:

For Men

  • Bioelectric Impedance: 43.9% defined as “obese.”
  • BMI Method: 19.1% defined as “obese.”

For Women

  • Bioelectric Impedance: 52.3% defined as “obese.”
  • BMI Method: 24.7% defined as “obese.”

These results bring us to the question: Is BMI accurate? Well, the answer isn’t a simple one. The evidence we have found would indicate that BMI is not a great indicator of overall body fat content. That being said, many people have found it to be a useful tool for managing their weight. We would say that BMI is something that you should use, but don’t expect it to be an exact reflection of your body fat levels.

Limitations Of The BMI Method

Some people have moved away from the use of BMI as an indicator. Critics of this method do have some valid points. The main problem with the BMI method is this: It only accounts for two factors, those being height and weight. There are quite a few other factors that can influence your weight, including muscle mass, age, gender, and ethnicity. All of these factors can influence weight to one extent or another, and BMI does not account for any of them.

Also, there are a number of studies that seem to indicate another problem: People frequently lie about their weight or height. In many other cases, they might simply be wrong because they haven’t measured their weight in a while. Take this one, for instance. In this study, the participants were asked to submit their height and weight in an interview. Based on those self-reported measurements, researchers calculated the BMI for each test subject. Then, they did the same thing again: Only this time, they put the participants on a scale and measured their height accurately. They found that almost half of the test subjects did not report their height and weight accurately and that those inaccuracies threw off the BMI measurements by quite a bit.

Let’s also consider one final factor: Muscle weighs more than fat. Because of this, athletes are regularly classified as “obese” on the BMI scale because they have a lot more muscle mass than the average person. Thus, BMI measurements are not very useful for those with a lot of muscle.


In the end, BMI measurements are a practical tool by which you can keep track of your progress. For those who are trying to improve their physical fitness, we would recommend the use of the BMI method. However, you should remember its limitations and adjust your view accordingly.

It is fair to say that the BMI method needs adjustment to account for the many different types of people in the world. We can clearly see that gender and ethnicity play a role, and certain medical conditions can undoubtedly make a difference as well. So, use this method, but understand it for what it is: A useful ballpark estimate. If you have found this article to be helpful, we invite you to follow us on Facebook for more of our work.

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