What is BMI?

BMI stands for Body Mass Index. It's a person's weight in kilograms divided by the square of their height in metres. The result sorts you into a weight status like this:

Look closer and the BMI formula has a lot in common with the formula for density. BMI is essentially a calculation of a person's density, but a flawed one.

Density is the mass of something divided by its volume. Cotton has a lower density than iron: a given volume of cotton weighs less than the same volume of iron. So BMI is trying to tell us how much a given volume of a person's body weighs. Is that interesting in any way?

And in the BMI calculation, the "volume" of the person is their height. How can height equal volume for a person? People are built differently. That means one of the two factors in the calculation is effectively worthless.

The second factor is a person's weight. Weight could be interesting for training and diet, if we knew what that weight was made of. Is it body fat, muscle, both, or something else? BMI doesn't mention that at all.

Arnold Schwarzenegger in the 1970s carried huge amounts of lean muscle and very little body fat. Because of all that muscle, his body weight was high, which put his BMI well above 30, labelling him "obese." Anyone who has seen pictures of Arnold from the 70s, 80s or 90s knows "obese" is nowhere near an honest description of his condition.

So both factors in the BMI calculation are close to worthless, and the answer it hands you isn't much use.

How can I use BMI to become healthier? How could it guide my training or my diet? In what way is it actually interesting?

I'm sorry, but I don't have good answers to any of those questions.

Let me put it this way: do not use Body Mass Index. Don't calculate it. Don't care about it. Pretend it never existed. Use a mirror instead, or just don't worry about it, lift some heavy weights, do some cardio, and enjoy getting in good shape.