What You Need to Know About Your BMI
This is Your Quick Training Tip, a chance to learn how to work smarter in just a few moments so you can get right to your workout.
I’ll never forget the time my doctor told me my weight was a problem. His exact words were “worryingly overweight,” and he uttered them while reviewing my BMI (body mass index), which a nurse had calculated after having me hop on a scale. In fact, he was so worried that he handed me a complimentary pamphlet on how to lose weight and why that would be important for my health. It had a lot of helpful illustrations.
That was the last time I visited that doctor—not because my ego couldn’t handle his diagnosis, but rather because it was so profoundly closed-minded. My BMI might have been 27, which landed me squarely in the overweight range—but at the time of my visit I could bench press more than 300 pounds and my body fat percentage hovered around 11 percent. In short, I was strong and cut as hell. My abs cast their own shadows.
That’s the problem with BMI calculations. They don’t take into account body composition (i.e., ratio of lean mass to fat mass). First introduced in the 1830s by a Belgian mathematician named Adolphe Quetelet, BMI uses a simple formula (weight in kilograms divided by height in meters squared) to assess a person’s body weight and divine their level of body fat. For adults, anything below 18.5 is considered "underweight," 18.5 to 24.9 is considered "healthy," 25 to 29.9 is "overweight," and over 30 is "obese." In other words, the higher your BMI is, the more flab you have—or so the thinking goes.
To be fair, BMI works fairly well for the general population—the majority of people don't meet the U.S. guidelines for healthy physical activity, meaning they're not training to build muscle—but if you’re lean and muscular, BMI is meaningless. The reason is that muscle weighs more than fat by volume, making it entirely possible to gain weight (and increase your BMI) as you build muscle and lose fat. So if you eat healthfully and strength train consistently, you’re better off using another method to gauge your “body comp.”
Your move: Focus on your body fat percentage. The most accurate measurement techniques are hydrostatic weighing (also called underwater weighing), air displacement plethysmography (similar to hydrostatic weighing, but with air instead of water), and DEXA (short for dual-energy X-ray absorptiometry). Unfortunately, these methods also tend to be expensive and available only at elite training facilities, independent labs, and advanced medical centers.
Alternatively, you can use a “body fat scale,” which uses a weak electrical current to determine body fat percentage. Other methods, like old school calipers for skin fold measurements, can also help you to get a rough estimate.
But body fat percentage shouldn't be the end all, be all number driving your training. Bodybuilders and elite athletes sometimes dip below 10 percent, but they rarely stay there. Your body needs a certain amount of fat to function properly. So aim for the low double digits, and be proud if you crack 15 percent, as that’s when you’ll achieve a level of fitness that most people only dream about. If you keep the focus of your workouts on achieving other real world goals, you'll be in a position to be in great shape, no matter what the scale says.
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