With all the talk about it these days, you might be wondering, “What is BMI?” There's no magic to it, no secret formula, even though sometimes the way it's talked about it sounds like some sort of mantra for weight loss and health. BMI isn't a meditation or an exercise or diet plan, it's simply the measure of your body mass. It compares your weight and height, and boils that ratio down to a number. Depending where your number is on the scale, it tells you if you're underweight for your height, overweight, or obese.
So what is BMI, in general? BMI, or body mass index, is a tool of body mass measurement that was created in the early 1800s by Adolphe Quetelet. This is why sometimes you might see BMI referred to as the Quetelet index. Adolphe Quetelet was mathematician, statistician, and an astronomer, among other things. He developed the BMI while working on social physics. The BMI is still used today with very few variations on his original invention.
What is BMI, specifically? BMI doesn't measure body fat, but instead compares a person's weight and height to estimate a healthy body weight for that person's height. It's widely used to determine if someone is technically overweight or obese. Though it's the not the final tool in diagnosing obesity or overweight, because it's not without its flaws for certain body types, it's still in use in the medical community.
The formula for BMI is simple: Take your height in meters squared (your height times itself) and then divide your weight in kilograms by that number. For example, if I'm 5 feet tall, that's about 1.5 meters. 1.5 squared (1.5 times 1.5) equals 2.5. If I weigh 120 pounds, that's about 54.5 kilograms. So I need to divide 54.5 by 2.5 which gives me a result of 21.8 as my BMI. So what does that number mean? A BMI of 18.5 or below indicates that an individual is underweight. The normal weight range is from 18.5 to 24.9. Overweight people will calculate a BMI of 25 to 29.9. A BMI of 30 or greater indicates obesity. Someone five feet tall weighing 120 pounds falls into the normal weight range.
Where BMI fails is with extremely muscular people. It doesn't distinguish between muscle and fat, instead using total weight as the measure. So according to the BMI, bodybuilders are likely to measure as obese which is inaccurate. Most athletes like wrestlers, footballs players and any athletes with a large amount of lean muscle are going to measure as overweight or obese. But for most people, BMI is a fairly accurate measure of their level of underweight or overweight.
Now that you know the formula, you know that BMI is as simple as a single math problem, and a single number. Next time you're faced with the question, “What is BMI?” you'll know there's no magic to it, it's simply a measure of a person's body weight compared to his or her height.