If you want to know what the BMI chart is, how to use the BMI formula, whether or not it’s accurate, and how it compares to other methods for measuring your health, then you want to read this article.
- Many people claim that the body mass index (BMI) chart is useless because it only measures body weight, and thus might classify people with more muscle as “overweight” or “obese.”
- At best, though, building muscle is only going to push you into the “overweight” category. If you’re obese according to a BMI chart, it’s almost guaranteed that you have too much body fat.
- If you’re “overweight” according to a BMI chart, then you can easily determine if you have too much body fat by measuring your body composition (tips on how below).
“BMI charts are bogus”
“BMI Is A Terrible Measure Of Health”
“5 BMI Myths You Need To Stop Believing”
Those are the titles of several articles on BMI published in the last few years. If you take them at face value, then you’ll come to one conclusion:
BMI is outdated, overrated, and inaccurate, and it’s time we retired it for good.
In case you don’t know, BMI stands for “body mass index,” and it’s a measurement of the relationship between your height and weight.
For decades, BMI has been used by doctors to help determine whether or not someone should consider losing or gaining weight, but recently, this idea has come under fire.
Detractors say that you can’t distill what it means to be healthy into a single number.
Others say that people who are “big boned” are going to be unfairly labeled as overweight according to their BMI.
And still others point out that BMI charts don’t work well for muscular people.
What are you supposed to think?
Well, the short story is that while BMI is imperfect, it’s still one of the best ways to get a quick bird’s-eye view of your body composition and overall health. As long as you understand it properly, the standard BMI chart is a decent barometer of whether or not you should lose weight.
I’ll explain why in a moment. First, let’s look at what BMI is and where it came from.
What Is BMI?
BMI is short for “body mass index,” which is a numeric expression of the relationship between your height and weight.
To calculate BMI, you divide your weight in kilograms by your height in meters squared.
After calculating your BMI, you use your score to determine what weight category you belong to:
Underweight = BMI of less than 18.5
Normal weight = BMI of 18.5 to 24.9
Overweight = BMI of 25 to 29.9
Obese = BMI of 30 or greater
These categories are sometimes modified for different groups of people. For example, some studies show that Asians have a higher risk of weight-related diseases despite being in the “normal weight” category, so the thresholds for what’s considered overweight or obese should be lowered for this group.
The equation for calculating BMI was developed in the early 1800’s by a Belgian mathematician named Adolphe Quetelet, who was one of the first to statistically analyze variations in human size, proportions, and growth rates.
Known as the Quetelet index, it was popularized as “body mass index” after the publication of a 1972 paper showing a link between BMI and health problems.
At the time, the idea that excess body fat was a major cause of poor health wasn’t airtight, and scientists needed a simple way to estimate the body fat percentage of large groups of people.
Thus, they decided to use BMI as a proxy for body fatness, which is how it’s still used today.
What Is a BMI Chart?
A BMI chart is a table or graphic that helps you easily find which weight category you currently belong to.
Here’s what they usually look like:
And here’s a visual of the same information:
To use a BMI chart, though, you first need to know your BMI.
Let’s calculate mine and see where I fall on the first BMI chart.
I weigh 180 pounds and I’m 5’10 inches tall.
First, we convert my weight into kilograms and my height into meters:
180 (pounds) / 2.2 = 81.8 (kilograms)
70 (inches) x 0.0254 = 1.78 (meters)
Next, we square the height measurement:
1.78 x 1.78 = 3.161
And finally, we divide the weight by the height squared to get my BMI:
81.8 / 3.161 = 25.9 (BMI)
We can round that to 26.
Next, we find my height in the vertical column of the table:
We trace the row to the right until we find my BMI.
According to the BMI chart, I’m overweight…
… or am I?
According to body fat calipers, I’m sitting at 12 percent body fat right now and have a 32-inch waste—not exactly Michael Moore territory.
Mike Matthews is another good example of someone who’s physique belies his BMI. If you run his numbers (6’2 inches tall, 195 pounds), his BMI is 25—just over the normal healthy range.
And he looks like this:
Studies do show that people with a higher than normal BMI have a greater risk of disease, though, so should we be worried?
Not really. Let’s go over why.
Is the BMI Chart Accurate?
Well, it depends… accurate for what and for whom?
If you take a large group of people and measure their BMI, you can get a decent estimate of how fat that group of people is on the whole.
If you take a single person, though, the results become more muddled.
According to most of the blogs, videos, and magazines you’ll find online, the fault of most BMI charts is that they overestimate the number of people who are overweight. That is, if you judge people solely based on their BMI, a lot of perfectly healthy people are going to be classified as overweight or obese.
The truth is that the main problem with BMI charts is that they underestimate the number of people who are overweight or obese.
A 2010 meta-analysis published in the International Journal of Obesity illustrates this nicely. Researchers looked at the BMIs of 32,000 people whose body fat percentage was also directly measured through other means.
Half of the people who were considered “normal weight” or “overweight” according to BMI were obese if you look at their actual body fat percentages.
In other words, BMI actually paints a rosier color of people’s health than many want to believe.
Things became even more clear cut when predicting who was obese or not. In the same study, the researchers found that 95% of men and 99% of women who were obese according to the BMI chart were, in fact, actually obese.
If you’re obese according to a BMI chart, you’re probably obese.
Okay, fine, but what about people who are just “overweight?” The BMI chart doesn’t measure muscle mass, so couldn’t these people just have more muscle than others?
And what about bone mass? Is it true that some people can just be “big boned”?
Won’t these people be unfairly labeled and pressured to lose weight?
Are BMI Charts Accurate for Muscular People?
Standard BMI charts are based on the assumption that the people being measured have a relatively normal amount of lean body mass.
For most people, levels of lean body mass remain stable, while changes in body composition come from rising or falling body fat levels. Therefore, if someone is well above the “normal weight” category on BMI charts, then it’s reasonable to assume that change is caused by a large increase in body fat.
The vast majority of people have, by definition, an average amount of muscle mass, which means BMI charts are going to be accurate more often than not.
What about people who have more muscle than average, though?
Couldn’t they be unfairly classified as “obese” according to BMI?
Even the most dedicated, genetically gifted of us can only build enough muscle to nudge us into the “overweight” category, which isn’t an issue as long as you’re still relatively lean (more on that in a moment).
For example, according to the best predictors of my genetic muscle-building potential, the most I’ll ever probably weigh is 190 to 195 pounds at 10% body fat. To get there, I’ll need to gain another 16 pounds of muscle.
That would peg my BMI at 27 to 28, which is right in the middle of the “overweight” category.
To be considered “obese,” I’d have to gain 31 pounds of muscle.
That simply isn’t possible without drugs.
Here are some more examples of people close to their genetic limit for muscle growth:
A woman who’s 5’4, 155 pounds, and 20% body fat would have a BMI of 26.5, which is in the middle of the “overweight” category.
A man who’s 6’2, 215 pounds, and 10% body fat would have a BMI of 27.5, again, in the middle of the “overweight” category.
This brings us to another specious argument that “BMI-deniers” like to harp on. They’ll often say things like “Just look at your favorite athletes! Many of them are in the peak of health but are considered obese according to BMI alone!”
Well, sure, but pretending that professional athletes aren’t using some extra #dedication to reach those freakish weights is, at best, optimistic.
That brings us to the final part of the argument against BMI—it doesn’t account for how bone mass affects your body weight.
Are BMI Charts Accurate for “Big-Boned” People?
Bone mineral density, or simply bone density, is a measurement of the strength and weight of your skeleton.
Thus, many point to this as another failing of BMI charts. The idea is that some people have such dense bones that they could end up in the “overweight” or “obese” category.
And you can see why with simple math.
Depending on who you ask, your skeleton is around 15% of your total body weight.
Around ~25% of that weight comes from water and fat.
The remaining 75% of your bone mass is composed of minerals like calcium. If one person is “big boned,” the idea is that they have bones that contain more minerals, thus making them weigh more.
There can be major differences in bone density between two people, but this makes almost no difference in your body weight, and thus BMI.
Here’s an example, using myself again.
I weigh 180 pounds, so we can assume my skeleton weighs around 27 pounds (15% of body weight, remember).
180 x 15% = 27 pounds.
We can subtract 25% from those 27 pounds to estimate the mineral weight of my bones.
27 x 25% = 20.3
Now, let’s say I increased my bone mineral density by 5%, which is the upper end of what you can expect from a good strength training program.
That makes my bone mineral weight, 21.3 pounds—a single pound heavier.
My total body weight would then be 181 pounds, which isn’t going to change my BMI to any meaningful degree.
Bone mineral density varies significantly from person to person. It can also change several-fold thanks to diet and exercise. But, the absolute weight of your bones simply isn’t enough to make any difference in your BMI. In other words, no one is “big boned” enough to end up in a higher category than they deserve on a BMI chart.
The Bottom Line on Why BMI Isn’t Bogus
In the final analysis, the chances are slim to nil that you’ll find yourself in the “obese” category without also carrying a lot of body fat or using steroids.
The real reason so many people like to rail against BMI is because it’s often a number they don’t like.
Instead of telling people the truth—that being obese is bad for you, and BMI is good at predicting who is obese—they point to the shortcomings of BMI as evidence that it doesn’t matter.
The fact is that if you’re obese according to BMI charts, there’s a 95% chance you’re actually obese if you’re a man and a 99% chance you’re obese if you’re a woman. The reason you’re considered “obese” according to a BMI chart isn’t because you have excess muscle or bone. It’s because you’re carrying too much body fat.
A Better Way to Know if You’re Healthy
BMI is based on your height and weight, and your height isn’t going to change.
So, in effect, tracking your BMI is no different than tracking your body weight.
That’s a good start for measuring your progress, but it doesn’t tell you anything about your body composition, which is what you want to improve.
Body composition refers to how your body breaks down in terms of fat, muscle, water, bone, etc. This is a much more accurate and helpful metric for gauging your health.
There are a few different methods and models for calculating body composition, but we’ll stick with a simple one for the purposes of this article—one that separates your body into two categories:
- Fat mass, which is all of the fat in your body.
- Fat-free mass (often referred to as FFM), which is everything in your body that isn’t fat, including muscle, bone, blood, organs, water, glycogen, and more.
By monitoring these things—as opposed to BMI or just body weight—we can better understand our fitness level and how quickly we’re moving toward our goals.
Estimating your body fat percentage isn’t hard, but since every method has its pros and cons, your best bet is to use several different techniques to judge your progress.
The best ones are:
- Body fat calipers.
- Waist circumference.
- Progress pictures and the mirror.
- How your clothes fit.
If you want to learn how to use these to track your body fat percentage over time, and how to use that data to improve your body composition, then check out this article:
The Bottom Line on BMI Charts
BMI isn’t a perfect measure of your health, but it provides a good snapshot of whether or not you should lose weight.
If you’re “obese” according to a BMI chart, then chances are good that you should lose weight.
It’s not because you’re “big boned” or overmuscled. It’s because you’re carrying too much body fat.
If you’re “overweight” according to a BMI chart, then you probably have too much body fat, or you’re getting close to your genetic limit for muscle growth.
Instead of focusing on your BMI, though, you’re better off tracking your body fat percentage.
This tells you what you really want to know, which is how much of your body is composed of fat versus muscle. And if you know what you’re doing with your diet and training, then you can drastically improve these numbers over time (even as your BMI remains more or less stable).
One of the best things you can do to control your body fat percentage is to manage your diet properly, and that starts with meal planning. If you want to learn how to make meal plans that work for any diet, without giving up your favorite foods, that guarantee results, then check out this article: