The Skinny on BMI

Doctors looking at a chart.

Most of us public health geeks probably know what a body mass index (BMI) is. As a refresher, it’s a number calculated based on a person’s height and weight. Although it seems like a simple concept, for lots of people, it isn’t.

For starters, it’s a calculation, which means you have to do math. Even if you use the chart that calculates it for you, reading the chart still requires math skills.

Then take its use: health professionals essentially use it as a “red flag” for weight issues. That means when you’re talking about BMI, you’re talking about risk — about what might be true.

And as if that’s not enough, it might not mean anything. That’s right, BMI is an accurate indicator for most — but not all — people.

So you can see, when you take a step back, BMI can be pretty confusing. And you know what else can be confusing? The chart we’re supposed to use to calculate it. Take a look:

BMI Chart

For people who have trouble with numeracy, which is most of us (public health professionals included), this chart is anything but easy to use. The color on this “simple” version helps, but try finding your BMI without touching the screen to follow the row and column — it’s not easy. And this is a simple version!

So the next time you find yourself mentioning BMI in a written material, you might want to make sure you’re explaining how and why BMI is used as a screening tool.

Oh, and let’s redesign that chart!

The bottom line: Proceed with caution when mentioning BMI. Be sure to explain what it means, and what it doesn’t. And don’t rely on your readers being able to understand the chart.