Should You Care About Your Body Mass Index?

By Korin Miller |

Find out what this number really says—and doesn’t say—about your body and health.

body mass index

You probably know how much you weigh, but whether the number on the scale means you’re healthy depends on many factors—and your height is just one of them.

Your body mass index (BMI) is a score that uses both your height and weight to provide a rough estimate of whether you’re underweight, normal weight, overweight, or obese.

Research has consistently shown that people with a high BMI have a greater risk of serious health problems, including heart disease, diabetes, and certain cancers. This makes sense, because most people with high BMIs are in fact overweight or obese.

But “most” is the operative word. BMI is an effective tool at tracking large populations over time, but what it means to each individual—to you—is where things get more complicated.

The Problems with BMI

BMI isn’t perfect. It doesn’t account for your bone structure, gender, genetics, or any conditions like osteoporosis. Nor does it actually tell you how much fat you’re carrying around.

Since muscle is denser than fat, it’s possible to be very fit and have a BMI in the overweight or obese category—though that’s most apt to happen if you’re an athlete or extremely muscular.

Older adults should also know that their BMI might appear normal when they would actually benefit from gaining some weight. That might be the case if you’ve lost a lot of muscle mass because of aging. In fact, some studies have found that elderly people who are overweight, according to their BMI, actually live longer than their thinner peers.

“BMI isn’t going to give you the whole picture, but it’s a piece of the puzzle,” says New York-based dietitian Jessica Cording, R.D.N.

In short: Consider BMI a tool, but not the final word on your health.

How to Calculate Your BMI

The easiest way is to use an online calculator, such as this one provided by the Centers for Disease Control and Prevention (CDC). All you need to do: Enter your height and weight, and click a button.

If you prefer the DIY method, you’ll need to use this formula:

BMI = [Your weight in pounds ÷ (Your height in inches)2] x 703

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Either way, you’ll be given your BMI, which is interpreted as follows:

  • Underweight = less than 18.5
  • Normal weight = 18.5 to 24.9
  • Overweight = 25.0 to 29.9
  • Obese = 30.0 or greater

What to Do with Your Number

Whatever your score, don’t ignore it. “BMI gives you some indication of lean versus fat mass,” explains Sonya Angelone, R.D.N., a spokesperson for the Academy of Nutrition and Dietetics.

Most people with a BMI in the overweight or obese range will benefit from losing some weight, and those who are underweight may need to gain weight. But before you make any major changes to your diet or exercise routine, talk to your doctor.

According to research published in the Annals of Epidemiology, waist-to-hip ratio might be a better gauge of obesity in people over age 70. Your doctor might want to check those measurements and use them along with your BMI and other health metrics (like your most recent blood test results) before deciding how to advise you.

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