First, it doesn’t take into account your lean body mass.

Let’s illustrate how this comes into play.

Say you have two men, both of whom stand 5’7” tall and weigh 195 pounds. The first man is very heavy set. He doesn’t do much activity at all and works an office job. As such, he has packed on a few pounds of fat over the years and would be described as quite robust.

The second man weighs the same amount, but standing these two men side by side, you see stark contrasts. The second man takes up much less space because the weight he has on his frame is dense muscle. He’s athletic, works out regularly and keeps his body fat to a low level.

Because muscle mass tissue takes up far less space compared to body fat tissue, the 195 pound inactive man is much larger than the 195 pound active man, for instance.

But yet, when you plug their numbers into a BMI calculator, both men come back with a BMI of 30.5, putting them in the obese category, for example. The RMR calculator is a perfect example for this

While the first man may in fact be considered obese, the second man is far from it. But yet, by medical standards, that’s what they place him in. He may only have a few pounds of fat on his frame, but yet, he’s classified the same as someone who’s easily 30% or more total body fat weight, for instance.

A BMI calculator does not give you an accurate representation of what your body weight composition is. Is it primarily fat weight? Or is it lean muscle mass weight? That makes a big difference in terms of your overall health as well as performance.

And, from a progress-tracking standpoint, you can also run into issues with a BMI weight loss calculator.