How to Calculate BMI (And Does It Matter?)

How to Calculate Your BMI (And Does It Matter?)
Photo: Pond5

You’ve heard it countless times: Maintaining a healthy weight equals fewer medical problems. But simply monitoring the scale may not be enough. That’s where body mass index (BMI), a measure of a person’s body fat based on their height and weight, comes to play. “BMI is a good way to assess if a patient’s weight is healthy or unhealthy,” says Jessica Crandall, RD, certified diabetes educator and national spokesperson for the Academy of Nutrition and Dietetics.

While your BMI doesn’t give a comprehensive evaluation of your health, it does excel as an easy-to-calculate method that can give you a basic look at your overall weight issues. But there’s a lot of weight to this debate — here’s why.

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BMI: The Full Story

To accurately measure your BMI, all it takes is a simple equation: Divide a person’s weight in pounds by the square of their height in inches, and multiply it by 703. At its core, it’s simple: The higher your weight, the higher your BMI number — and the higher your risk of obesity-related diseases, says Nicolette Pace, RDN, President and Founder of NutriSource Inc., Medical Nutrition and Weight Loss Center. Doctors, researchers, even insurance providers use the formula as a primary measure of health. And it’s a simple and inexpensive screening tool that people can check from the comfort of home.


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A BMI number places people in categories — underweight (<18.5), normal (18.5-24.9), overweight (25.0-29.9) and obese (>30). Over the past 20 years, BMI categories have seen major changes. In 1998, the National Institutes of Health and the Centers for Disease Control and Prevention lowered the threshold for the ‘overweight’ category to 25 from 27.8, to match World Health Organization guidelines. When the changes were made in 1998, 29 million Americans previously considered to be “healthy” were immediately labeled “overweight.”

How to Calculate Your BMI (And Does It Matter?)
Photo: Pond5

The Great Weight Debate

While doctors often use the equation to identify potential weight problems in patients, BMI is only telling part of the story. The method falters by not taking muscle mass, cholesterol, blood sugar, blood pressure or waist circumference into consideration. Experts say waist circumference, specifically, is a major element being ignored. A higher ratio of fat around your waist compared to your hips puts people at a greater risk for heart disease and type 2 diabetes. “Waist circumference, while not better than testing BMI, is another parameter that’s needed to interpret health risk,” says Pace.

Another big problem: muscle. “BMI is less accurate for individuals with more muscle since it doesn’t take into account muscle versus fat; it just considers total body weight,” Crandall says. “For bodybuilders or individuals who have extremely high muscle mass, it may not be a good [method], like it is for the average athlete or fit person.” That’s because those with more muscle are carrying around extra weight, Crandall adds. Yet, “For most people that are ‘fit,’ they’ll still fall into the correct BMI category that their body represents,” Crandall says.

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There are also emotional reasons why funneling people into strict categories of ‘overweight’ or ‘underweight’ may not be a good idea. “BMI labels are objectionable and help to keep the obesity stigma alive, many times interfering with treatments, fostering negative body image and unrealistic standards,” says Pace. She recommends looking at waist circumference, frame size, body fat percentage, weight history and any acute or chronic changes in addition to just BMI.

The Big Picture: Beyond BMI

Most experts agree that BMI is a weighty matter — and it’s far from perfect. But, while BMI only scratches the surface when it comes to highlighting the potential health risks of being under or overweight, it doesn’t hurt to use it as a base overview of your health. “BMI is mostly a useful indicator to relay diagnostic information,” says Pace. “But I try to get my patients to look at the bigger picture and get to the underlying causes of their weight problems and health conditions.”

Looking beyond the scale at muscle mass, level of exercise and waist circumference can help give a more comprehensive look at how fit or healthy you truly are. Take advantage of the BMI calculator for a quick at-home peek at your weight — and then dig deeper with a doctor to make sure you’re on the right path for your body. “BMI is a useful tool and point of reference when used in conjunction with other monitoring tools,” says Crandall. “Everyone’s body is different so it’s not perfect, but it does serve as a good, general way to categorize and monitor patients.”

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