Body Mass Index is as widely used as it's misunderstood. People are fighting over whether to use BMI or not but neither side seems to understand that they're actually both wrong.
BMI is a way to measure overweight and obesity, as a marker for health. It doesn't measure actual fat though, but focuses on a more easily measured proxy – weight in relation to height.
This is a quick and efficient approach. Excellent for its intended purposes!
Those purposes, however, don't include you and me and so it's practically worthless for us.
The formula to calculate BMI with the metric system is:
Weight (kg) / Height (m)^2.
And with the Imperial system:
(Weight (lbs.) * 703) / Height (inches)^2.
If your resulting Body Mass Index is below 18.5 you're underweight, 18.5 - 25 you're normal weight, 25 - 30 you're overweight, and 30+ you're obese.
Except this is very misleading. Don't pay these numbers any mind. Unless you're a scientist.
Adolphe Quetelet (1796-1874) was a Belgian mathematician, astronomer, and statistician.
(Just imagine the guy's LinkedIn profile.)
Adolphe devised the so-called Quetelet Index in 1832, as a tool for tracking weight status in populations.
More than hundred years later, in 1972, the Quetelet Index was renamed to the Body Mass Index.
The usage is the same though. You take a group of people, run their height and weight through the above formula, average the result, and then use that number to determine the status of the group.
BMI for individuals is so blunt and crude it's useless for individuals but for bigger groups it's great. It can, for example, warn us about rising child obesity.
Now that you know not to use BMI for yourself – what should you use instead?
If you want to know exactly how much fat you're carrying around, to plan for a cut, then there's a wide selection of tools and methods for that. I recommend getting a plastic caliper. They're cheap, easy to learn, and fairly accurate. If you can afford it, a bod pod or dexa scan are even more accurate.
However, if your main interest is simply knowing whether you're overweight or even obese, then you probably already have all that you need at home. It's a simple three-part procedure:
- Strip down to your underwear.
- Step in front of your bathroom mirror.
- Say after me: I can squat a shitload of weight
If you can't squat a shitload of weight yet, you simply have some more work to do.
The point is that you probably already know whether you're fat or not. Worrying about it makes no difference, so stop that and start focusing on the weight of your barbell instead.
The rest will take care of itself.