# In Defense of BMI—Sort Of

## Is Body Mass Index a perfect measure? No. But despite what the critics say, it’s a number that is valuable to you.

| September 12th, 2013

Body Mass Index, usually just called BMI, is a simplified way to figure out if your weight is healthy, too low or too high. Some people argue that it’s too simplified to be meaningful. We see their point, but we also think the critics are overlooking some info that is important for you. Normally we’re not advocates of shortcuts to health, but this is an exception. That’s because imperfect as it may be, knowing your BMI is like pulling a “Skip 10 Spaces” card in the board game of good health.

First, the basics: The formula used to calculate BMI compares your weight to your height and gives you a number, which you match to a scale that goes from underweight to obese. (The actual formula is weight in pounds / height in inches squared x 703, but we think it’s easier to take our Body Health Quiz and let us do that math for you.) A BMI from 18.5 to 24.9 is ideal. Below 18.5 is underweight, 25 to 29.9 is overweight and 30 or above indicates obesity.

MORE: Is BMI Underestimating the Obesity Epidemic?

Now, that’s how it works in a perfect world. But we all know that we don’t live in a perfect world. If you have big, dense bones, for instance, you’re going to weigh more than other people your height with smaller, lighter bones. As a result, you might fall into a higher BMI category than someone with your body fat percentage should. Similarly, athletes who are all muscle and almost no fat are going to have high BMIs—muscle is heavier than fat, so people with a lot of it weigh a lot. In fact, one study found that more than half of the players in the NFL are “obese” according to their BMIs. That’s probably not the case. The calculator can’t tell the difference between a jacked-up linebacker and a person who’s just as tall and carrying too much belly fat.

But you can.

You are the missing part of the equation. You, unlike that formula, have eyes and can think critically. If your BMI indicates you’re overweight or obese, yet you can’t pinch an inch of fat off your rock-hard six pack, then you don’t need to worry about what the calculator says. But most of us don’t have that kind of body (and those who do probably don’t need to be online calculating their BMI). Most of us have noticed our bellies get bigger as we’ve aged. Or maybe we’re just not sure whether we’re “normal.” That’s where BMI comes in handy.

If you’re not sure whether your weight is optimal for health, finding out where you fall on the BMI scale is a good place to start. Should you wind up getting a result that isn’t comfortably in the range between 18.5 and 24.9, it could be a sign that it’s time to assess your lifestyle. Start by talking to your doctor about your weight. He or she might even send you to do an impedance test (no radiation) to accurately measure your body-fat percentage, review your overall body composition based on a DEXA scan (often done to asses bone mass), or simply recommend an exercise and nutrition regimen that will help nudge you squarely into the healthy zone.

We know that you’re more than the numbers on the scale—whether that’s the scale in your bathroom or the BMI scale. But any number that puts you on track to better health is Number 1 in our book—and yours, too, we hope. It’s time to take charge for you.

QUIZ: Measure Your BMI and Body Health

Smart is sexy - get our newsletter: