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Here’s What You Need to Know About BMI and Body Fat
by Danielle Kosecki

Am I fat? If you’re a health-conscious person, this may be a question that’s crossed your mind. And not necessarily for vanity reasons: Nearly every major health organization, from the Centers for Disease Control and Prevention to the World Health Organization, has labeled obesity a disease and linked it to a variety of health conditions, such as depression, type-2 diabetes, and even certain cancers. So, it makes sense that you’d want to gauge your risk by quantifying exactly how “fat” you are. 

To do so, many people turn to body mass index (BMI), calipers, and scales. And that’s great: These at-home tools can be quick and easy ways to estimate your body fat percentage. However, your BMI and estimated body fat percentage should not be the only pieces of information you consider. 

Here’s why: These methods are designed to be screening—not diagnostic—tools, which means  they can sometimes produce conflicting information. For instance, you could find out that your weight is stable, your BMI is healthy, but your body fat percentage classifies you as obese. Or your weight could be stable, your body fat percentage good, but your BMI registers as obese. 

How do you make sense of those numbers? And how should you use them to get a better picture of your overall health? Step one: Learn a bit more about these body fat calculations.

What is BMI?

BMI is measure of body fat that’s calculated by dividing your weight (in kilograms) by the square of your height (in meters). For adults 20 years old and older, that number is then categorized into a weight status. 

BMIWeight Status
Below 18.5Underweight
18.5 – 24.9Normal or Healthy Weight
25.0 – 29.9Overweight
30.0 and AboveObese

“BMI does not measure body fat directly,” according to the CDC, but research shows that it moderately correlates with more direct tools, like skinfold thickness measurements, bioelectrical impedance (what your scale likely uses), and underwater weighing. 

However, despite the good correlation between BMI and body fat percentage, a Mayo Clinic study found that BMI tends to under-report obesity, missing more than half of people with excess fat. 

“People use BMI as a simple surrogate for weight, and most think they’re are supposed to have one that’s 23, which they refer to as ‘healthy,’” says Yoni Freedhoff, M.D., an American Board of Obesity Medicine-certified doctor and the founder and medical director of Ottawa’s Bariatric Medical Institute, which focuses on non-surgical, long-term weight management. 

But “BMI is a literal measure of bigness. It doesn’t speak to why a person’s big, it doesn’t take into account race, age, sex, or musculature. Even if you were to consider all of those things it would still be a highly flawed measure because scales don’t measure the presence or absence of health.” 

In other words, not everyone who is “big” is obese—this is why many professional football players get misclassified—and not everyone who is big is sick. 

A 2016 study in the International Journal of Obesity backs this up. When researchers compared the BMI of more than 40,000 people to six other heart-health metrics, like blood pressure and cholesterol, they found 47 percent of the people who were classified overweight, 30 percent of the people who were classified obese, and 16 percent of the people who were classified extremely obese were metabolically healthy—their other health stats were normal.

On the flip side, 30 percent of normal-weight people were at risk for heart trouble.

What is Body Fat Percentage?

Body fat percentage represents how much of your body’s total mass is made up of two types of fat: essential and storage. 

Essential fat is found in tissue, bone marrow, and organs, and it helps keep body processes humming. It’s believed that women tend to have more essential fat than men (around 12 percent versus 3 percent) to support childbearing and hormonal functions. 

The second, storage fat, protects organs but also can accumulate (around organs and directly under the skin) when more calories are consumed than burned off. You’ll often hear this type of fat referred to as visceral fat and subcutaneous fat, respectively.

20–39 yrs20–39 yrs
Underfat: <21%Underfat: <8%
Healthy: 21–32%Healthy: 8–19%
Overfat: 33–38%Overfat: 20–24%
Obese: >38%Obese: >24%
40–59 yrs40–59 yrs
Underfat: <23%Underfat: <11%
Healthy: 23–33%Healthy: 11–21%
Overfat: 34–39%Overfat: 22–27%
Obese: >39%Obese: >27%
60–79 yrs60–79 yrs
Underfat: <24%Underfat: <13%
Healthy: 24–35%Healthy: 13–24%
Overfat: 36–41%Overfat: 25–29%
Obese: >41%Obese: >29%

Underwater weighing is the gold-standard method for measuring body fat percentage, says Freedhoff. Energy x-ray absorptiometry (DEXA) scans are also accurate. But these clinical tools aren’t available to, or feasible, for most people.

Calipers and scales, which estimate body fat percentage, are convenient for home use but can be influenced by other factors such as hydration levels or for women, being premenstrual. So that’s why it’s important to look at your trends over time.

The Healthiest Way to Use BMI and Body Fat Percentage

While your weight can fluctuate daily, it takes a significant amount of time to see changes in your body fat calculations. So setting a weight goal is great, but if your real goal is to lower your your BMI or body fat percentage, just know you’re not going to get instant gratification. Body fat calculations can be useful pieces of information, says Freedhoff. “But when you use those measurements as the sole means to determine how you’re doing, you run the risk of being so disappointed that you quit,” says Freedhoff.

His philosophy? Live the healthiest life you can honestly enjoy living. Think about it as you would school, he says.

“The best grade an individual student is going to get is the grade they get when they go to class, do their homework, and study,” says Freedhoff. “But not every student who goes to class, does their homework, and studies is going to get an A+. And so, the goal is your best effort, and whatever happens, happens. And it’s great!”

But that doesn’t mean you should never step on a scale again. If you’re not a food logger, then over time weighing yourself and checking in our body fat percentage can give you a sense of whether you’re taking in more calories than you’re burning off. 

“If patients aren’t keeping track of what they’re consuming, I tend to recommend that they weigh themselves regularly to protect against unexpected weight gain,” says Freedhoff. “Make it naked, after pee, before breakfast, on Wednesdays.” Doing so increases your chances of getting consistent results. It buffers the weekend—when you’re much more likely to have been indulging or eating high-sodium foods that can cause water retention—and prevents day-to-day changes from messing with your head.

And remember, these numbers are just one of many tools. “If you suddenly step on a scale and you determine that you’re heavier than you’d hoped or your body fat percentage is higher than you’d hoped,” says Freedhoff, “these numbers aren’t going to determine whether you can live a healthier life and still enjoy it.”

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This information is for educational purposes only and is not intended as a substitute for medical diagnosis or treatment. You should not use this information to diagnose or treat a health problem or condition. Always check with your doctor before changing your diet, altering your sleep habits, taking supplements, or starting a new fitness routine.