Body mass index, BMI for short, is a controversial health metric. BMI has long been used by doctors to define obesity and many insurance companies even incorporate BMI into physicals in order to generate health or life insurance rates. But is BMI really that accurate at defining obesity? Let’s discuss.

What is BMI?

Body mass index is simply a measure of how much mass you carry. Calculating BMI is pretty simple and just takes into account your weight and height. To calculate your BMI, you simply convert your body weight from pounds to kilograms and your height to meters. Then, square your height and divide your weight by that number.

Generally, a low BMI is anything under 18.5, while a normal BMI is between 18.6 and 24.9. The overweight category sits between 25.0 and 29.9, and obese level 1 starts at 30. Obese level 2 sets in at 35. (Some agencies might refer to this as “extremely obese” or even “morbidly obese.” They’re not always shy with their words.)

The important distinction to make when considering BMI is that it’s “BODY mass index” rather than “FAT mass index.” This means that BMI cannot differentiate between muscle, fat, bone, water, Cheetos, or anything else in your body. All of your weight gets lumped together.

How Accurate is BMI?

The fact that BMI lumps all body weight together into one category is the reason for all of the controversy around it.

For sedentary populations, BMI isn’t a terrible proxy of overweight or obese status. If you don’t carry additional muscle from working out in the gym or years as an athlete, BMI is decently accurate at predicting whether or not you’re overweight. 

However, if you hit the gym regularly or you have a history as a competitive athlete, BMI is pretty pointless for you.

You see, the old adage “muscle weighs more than fat” is both true and false at the same time. Gravity affects everything the same; a pound of muscle weighs just as much as a pound of fat. However, muscle is more dense than fat. Thus, if someone carries a good deal of muscle, odds are they’re going to be dense a.k.a. heavy.

Time and time again, studies have shown that BMI consistently classifies athletes as overweight or obese when they’re actually far from either. Heck, most physique competitors, who can have single-digit body fat, (which is crazy low!) would be considered obese by BMI standards, all due to simply being heavier.

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A very insightful study published in Medicine and Science in Sports and Exercise displays this well. The scientists looked at a total of 226 male and female collegiate athletes from a variety of sports, as well as 213 nonathletes. When they calculated all participants’ BMIs and measured their body compositions, 67 percent of the male athletes were falsely identified as overweight by BMI, despite having a healthy body fat percentage. However, BMI was accurate for 75 percent of nonathlete males. Meanwhile, BMI was inaccurate for 31 percent of female athletes but just seven percent of nonathletes.

The study’s takeaway? That BMI is not an accurate measure of body fat in athletes, especially male athletes. 

Now, it’s important to have a moment of reflection here. Some folks might consider this a great excuse to ignore their BMI by just assuming they have a bunch of muscle. Be real with yourself, though. If you don’t work out at all and never really have, your BMI is probably decently accurate in regard to your actual obesity level. Always consider health metrics in the context of yourself and your goals.

What’s A Better Measure Than BMI?

Ideally, body composition should be measured by body composition measurement tools. The gold standard is the dual x-ray absorptiometry (DXA) device, but those cost tens, if not hundreds, of thousands of dollars. They also require a licensed operator—and you typically need a medical diagnosis that justifies being measured in one. Not exactly the most useful system for the majority of people!

Luckily, advances in technology have led to the creation of many smart scales that can estimate your body composition via bioelectrical impedance. This is a fancy way of saying that the machine sends a tiny electrical current through your body and measures how quickly it travels through tissue. It can then use this information to estimate your body composition. While it’s not the most accurate tool, it’s reasonably priced, quick, and completely painless—and you can use it in the privacy of your own home.

You can make bioelectric impedance a little more useful for you if you consistently use the same device and measure your body composition at the same time of the day and under the same conditions. For most folks, I’d recommend hopping on their scale on the first day of every month the minute they roll out of bed. This ensures that you have no additional food or water rolling around in your system that might bamboozle the machine. If you want to be extra accurate (like me), you can use the device on the first, second, and third days of the month and average your results to account for any small variations.

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If you don’t feel like dropping 50 to 100 dollars on a decent smart scale, many gyms now have bioelectric devices. Alas, it might cost extra to use these tools—or you might even have to sign up for personal training sessions before using them—so make sure you know what the deal is before you hop on a machine at your gym. Someone there will probably also record your results, and some folks might rather that information remain private. In that case, forking over a chunk of cash for your own device isn’t the worst idea.

Conclusion

Always keep in mind that BMI is not a measure of body composition (how much lean mass versus fat you have). Instead, it’s simply a way of describing how much weight you carry relative to your height. For sedentary people, it might be reasonably indicative of being overweight or obese. However, for athletes or people who work out often, BMI is worthless.

If you’re wanting to get a more accurate basis of your body composition, bioelectric impedance devices are perfectly suitable for most people—and you can improve the relevance of these tools by ensuring you always measure yourself under the exact same conditions.

Known as ‘The Muscle Ph.D.,’ Dr. Jacob Wilson has a knack for transforming challenging, complex concepts into understandable lessons that can support your body composition and health goals. A skeletal muscle physiologist and sports nutrition expert, Wilson is a leader in muscle sports nutrition. As the CEO of The Applied Science & Performance Institute and researches supplementation, nutrition, and their impact on muscle size, strength, and power.