- BMI stands for body mass index (BMI), and it’s a measure of your body weight per unit of height (calculated by dividing your weight by your height squared).
- Many people claim that BMI charts are useless because they only measure body weight, and thus might classify people with more muscle and bone as “overweight” or “obese.”
- The truth, though, is that if you’re obese according to a BMI chart, it’s almost guaranteed that you have too much body fat.
Already familiar with BMI and just want a BMI calculator to figure out where you stand?
Here you go:
Not sure what BMI is, how it’s calculated, or why it matters? Keep reading!
“One of the most popular ways of telling if you’re a healthy weight is bogus”
“BMI Is A Terrible Measure Of Health”
“5 BMI Myths You Need To Stop Believing”
These are titles of several articles on BMI and BMI charts published in recent 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BMI is outdated, overrated, and inaccurate, and it’s time we retired it for good.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 didn’t know, BMI stands for “body mass index,” and it measures the relationship between your height and weight.
BMI has been used by doctors for decades to help determine whether someone should consider losing or gaining weight, but recently this idea has come under fire.
Detractors say you can’t distill what it means to be healthy into a single number.
Some go further and say being overweight isn’t unhealthy, so why bother measuring BMI or worrying about your weight?
Others point out that BMI charts don’t work well for muscular people.
The truth is that BMI charts are imperfect, but they’re 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, BMI is a decent barometer of whether or not you should lose weight.
It’s not the only metric you should use to monitor your health, though, and we’ll get into some other methods in a moment.
Let’s start by looking at what BMI is.
Table of Contents
BMI stands 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.
For example, I weigh 180 pounds and am 5’10.
I first divide my body weight in pounds by 2.2 to convert it into kilograms.
180 x 2.2 = ~82 kilograms
Next, I divide my height in inches (70) by 39.37 to convert it into meters.
70 x 39.37 = 1.78 meters
Next, I square my height.
1.78 x 1.78 = 3.12
Finally, I divide 82 by 3.12 to arrive at my BMI.
82 / 3.12 = 26.28
We’ll round that to 26.
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
(As you can see, I’m part of team #overweight. This is misleading, though, for reasons we’ll go over in a moment).
These ranges are sometimes adjusted 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 some experts recommend lowering the threshold for what’s considered overweight or obese for Asians.
The equation for calculating BMI was developed in the early 1800s by a Belgian mathematician named Adolphe Quetelet, one of the first to statistically analyze variations in human size, proportions, and growth rates.
One of his most enduring accomplishments was creating one of the first formulas for estimating body fatness.
Known as the Quetelet index after its creator, this formula later became known as body mass index after the publication of a 1972 paper showing a link between BMI and health problems.
At the time, body fat was considered unsightly but not necessarily unhealthy.
As our understanding of the body leaped ahead in the 19th century, though, it became obvious that being fat—or corpulent in the parlance of the times—was a direct cause disease and dysfunction.
Scientists needed a simple way to estimate the body fat percentage of large groups of people to confirm their suspicions, and began using BMI as a proxy for body fatness, which is how it’s still used today.
A BMI chart is a table or graphic that helps you find which weight category you belong to based on your BMI.
Here’s what BMI charts look like:
And here’s a visual of the same information:
To use a BMI chart, though, you first need to know your BMI.
Based on the calculations we did a moment ago we know my BMI is about 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?
Body fat calipers show I’m sitting at 12 percent body fat right now and I 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 (6’1 inches tall, 195 pounds), his BMI is 25—just over the normal healthy range.
And he looks like this:
B-b-but research shows that we have a significantly higher risk of disease if we get skinnier.
Should we be worried?
Not really. Let’s go over why.
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.
For example, Mike and I are both overweight according to BMI charts, so it must be flawed, right?
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 conducted by scientists at the Mayo Clinic illustrates this nicely.
The scientists looked at the BMIs of 32,000 people whose body fat percentage was measured with bioelectrical impedance, which isn’t always accurate but works well enough for large groups of people.
Half of the people who were considered “normal weight” or “overweight” according to BMI were found to be obese based on their body fat percentage (25% body fat for men and 35% for women).
In other words, BMI actually paints a rosier picture of people’s health than many realize.
This is probably because these people were “skinny fat.” They had remarkably little muscle mass—which weighs more than fat—and a lot of body fat, which kept their total weight lower than you’d expect based on their level of fatness.
Although BMI wasn’t entirely accurate at determining whether people were overweight or not, it was very accurate at determining who was obese or not.
The scientists found that 95% of men and 99% of women who were obese according to the BMI chart were, in fact, actually obese.
Because when you gain enough body fat, BMI will predict your body fat percentage well enough to classify you as obese, regardless of how much or little muscle you’re carrying.
If you’re obese according to a BMI chart, you’re probably obese.
Okay, fine, you might be thinking—usually it’s not hard to tell if someone is obese or not.
But what about the exceptions? What about Mike and I? What about people who are big-boned?
Won’t these people be unfairly labeled as fat and pressured to lose weight?
The bottom line is that BMI charts usually underestimate how fat people are, especially if they have very little muscle and a high body fat percentage. BMI charts accurately determine whether or not people are obese.
Standard BMI charts are based on the assumption that the people being measured have a relatively normal amount of lean body mass.
In case you aren’t familiar with the term, lean body mass refers to all of the material in your body that isn’t fat, such as water, bone, and muscle.
People who don’t lift weights generally have relatively stable levels of lean mass, and so changes in body weight come from rising or falling body fat levels. If someone in this category is well above the “normal weight” category on BMI charts, then it’s reasonable to assume they’re there because they have a lot of 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 for them more often than not.
What about people who have more muscle than average?
What about people who lift weights?
Couldn’t they be unfairly classified as “obese” according to BMI charts?
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 pounds at 10% body fat. To get there I’ll need to gain another 15 pounds of muscle.
That would peg my BMI at 27, which is right in the middle of the “overweight” category.
To be considered “obese,” though, 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, 150 pounds, and 20% body fat would have a BMI of 25.7, which is in the middle of the “overweight” category.
A man who’s 6’2, 205 pounds, and 10% body fat would have a BMI of 26.3, 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 asinine.
The bottom line is lifting weights can give you enough additional muscle mass to be considered overweight according to BMI charts, but you can’t gain enough muscle to be considered obese without using steroids.
Bone mineral density, or simply bone density, is a measurement of the strength and weight of your skeleton.
Many point to this as another failing of BMI charts. Some people have such dense bones, they claim, that they could end up in the “overweight” or “obese” category.
You can see why with simple math.
Depending on who you ask, your skeleton weighs around 15% of your total body weight.
Around ~25% of that weight comes from water and fat.
What this means is that bone mineral density is going to have a small impact on your absolute body weight.
I’ll use myself as an example again.
I weigh 180 pounds, so we can assume my skeleton weighs around 27 pounds.
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 make a lick of difference in my BMI.
The bottom line is some people do have heavier bones than others, but the differences are too small to have a significant effect on your BMI.
Although women carry more essential body fat than men, it’s not a big enough difference to invalidate BMI charts.
A common criticism of BMI is that because it’s been around so long—the mid 1800s—it’s “outgrown its utility” and no longer represents a valid way to know if you’re at a healthy weight.
Like the iron lung for polio, heroin for the common cold, and lobotomies for seizures, BMI is just another outdated medical practice that should be thrown on the scrap heap of history, they say.
Last time I checked, though, bodily proportions and the fundamental rules of math haven’t changed much in 200 years, and the scientific evidence shows they certainly haven’t.
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 home truth is that the 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’s 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.
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.
And as you learned a moment ago, the main factor that leads to changes in body weight is changes in body fat.
That said, BMI doesn’t directly measure your body composition, which is what you really want to improve.
Body composition refers to how your body breaks down in terms of fat, muscle, water, bone, etc, and is a much more helpful metric for tracking your progress.
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 or lean body mass), 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 health and how quickly we’re moving toward our physique 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 get a more accurate reading.
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 tools to track your body fat percentage over time, and how to use that data to improve your body composition, then check out this article:
BMI stands for body mass index (BMI), and it’s a measure of your body weight per unit of height (calculated by dividing your weight by your height squared).
BMI is usually expressed in the form of BMI charts which divide people into four categories based on their BMI:
- 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
Although these categories aren’t a perfectly accurate measure of your true body fat percentage, they provide a good snapshot of whether or not you should lose weight.
BMI charts usually underestimate how fat people are, especially if they have very little muscle and a high body fat percentage. If you’re “obese” according to a BMI chart, then you can benefit from losing weight.
Lifting weights can give you enough additional muscle mass to be considered overweight according to BMI charts, but you can’t gain enough muscle to be considered obese without using steroids.
Some people do have heavier bones than others, but the differences are too small to have a significant effect on your BMI.
Instead of tracking your BMI, you’re better off tracking your body fat percentage.
The most important first step in controlling your body composition is managing your diet properly, and that starts with meal planning.
Check out this article to learn how:
Do you have a different take on BMI charts? Let me know in the comments below!