Your Questions About BMI, Answered

There’s a lot of hype about BMI (body mass index), but what does it really mean in terms of determining a healthy weight? How is it calculated? Is it accurate? Does it have shortcomings? The answers might not be what you’d expect.

What is BMI?

BMI is a simple method for determining the health of an individual’s weight based on his or her height. The technical calculation is mass in kilograms divided by height in meters squared. However, there are easy-to-use BMI calculators on the Internet that allow you to input pounds and inches (a link is provided at the end of this article). After you plug in your weight and height, the calculator gives you a number — this is your BMI.

BMI categories:

  • Underweight = less than 18.5
  • Normal weight = 18.5 – 24.9
  • Overweight = 25 – 29.9
  • Obese = 30 or greater

How seriously should BMI be considered?

While BMI is the preferred indicator of healthy weight, it certainly has its limitations, primarily that it does not tell you what percentage of body mass is truly body fat. This means that an athlete’s BMI could be misleadingly high due to increased muscle mass, and an infirm person’s BMI could be deceptively low due to muscle atrophy. And finally, adult BMI calculators should not be used for children because the amount of body fat changes with age and gender in those under 19 years.

Studies have also shown that waist circumference is a more accurate indicator of health risks than simply looking at body mass. That said, physicians grow concerned when BMI is 30 or above — it is rare to find a person with that number who is still healthy. This increases your risk for a myriad of health problems including:

  • High blood pressure
  • Diabetes
  • Heart disease
  • Breathing problems
  • Certain cancers

How should BMI be used in a weight loss program?

BMI is best suited to determine, with the help of your primary physician, your current health risks and then to establish your goal. For instance, if your number is 30 or above, your very first goal should be to get that number below 30. With the help of a BMI calculator, you can determine what weight will lower your BMI into a safer range. That is the weight you should set as your goal.

BMI should not be used to chart your progress — use a traditional scale for weekly monitoring. Because of the way it’s calculated, the BMI number will hardly change even when you’ve lost some weight, and you may wind up discouraged.

BMI is a good start, but it’s not the final word.

Your physician is the best resource to interpret BMI within the context of your physical makeup and overall health. Please call on him or her if you have any concerns about your weight. In the meantime, here are two additional resources:

BMI calculator (for adults)

Assessing your child’s weight

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