As if swimsuit season didn’t draw enough attention to your body, we thought it important to bring up another potentially sensitive subject: your body mass index, or “BMI.” But we’re doing so to let you know that this number, a derivative of your height and weight, isn’t all that and a bag of potato chips. Rather, it’s merely a number that can be helpful – or not. If you know the history, and how to interpret it, it’s not all that scary. Let’s dig in!
 
To calculate your BMI, first you have to convert your weight in pounds to kilograms. Then divide that by your height (in meters), squared. Alternatively, you could divide your weight in pounds by your height in inches (squared), then multiply by 703. Or, you can use an online calculator :) The number you get puts you into one of four categories: underweight (under 18.5 kg/m²), normal weight (18.5 to 25), overweight (25 to 30), or obese (over 30).
 
Ok, so if you’ve done the math, you now know your BMI. But what exactly does that tell you? Are you in an underweight or obese category but don’t think you should be? You’re not alone. The BMI is a number that’s helpful for population studies, but not necessarily individual health. Let’s explain.

A History

The BMI formula was developed by mathematician and astronomer Adolphe Quetelet in the 19^thcentury whose work centered on creating a picture of the “ideal” or “average” person. He came up with the BMI for his own purposes – so the figure wasn’t even created to be a reflection of individual health. Starting in the 20^th century, insurance companies began noticing their heaviest clients having the highest death rates, so finding a way to quantify individuals to determine their insurability became important. And thus, they started the cascade of everyone from epidemiologists to your family doctor, to now you, calculating your BMI.

What Doesn’t BMI Tell You? 

It doesn’t tell you how healthy you are at any given moment. It doesn’t tell you how much muscle mass you have. It doesn’t tell you how much fat mass you have, or where that fat is distributed on your body. Quite simply, all it tells you is where your height and weight fall on a spectrum. That means an Olympic athlete can be classified as “obese,” but a couch potato as “normal” or “healthy.” We know, of course, the athlete is in better shape. 

BMI Can Still Be Useful

So, let’s say you calculate your BMI today, and then in ten years, it’s higher. Assuming you’re done growing in height, that tells us you’ve gained some weight, which many people do as they age. But weight gain in adulthood (if the gain is in fat tissue alone), can indicate a higher risk for chronic diseases. And you may be feeling sluggish and unhappy anyway. So, this increase in BMI might be telling you something you already know: you’ve got to prioritize your health a little more. Or, it can tell you you’re now in a healthy weight range where you might have once been underweight. Or, conversely, that your healthy baby steps are paying off!

Bottom Line 

Tracking your BMI can be important, and can be an important step in calling a spade a spade. That is – if you know you need to work out more, then work out more. If you know you should cut down on the nightly cocktails or daily dessert, then do that. But don’t let it trip you up if you’re already living a healthy lifestyle. And if you need help stopping overeating, know that you’re not at all alone, and there are resources to help.