Why does BMI have units of kg/m^2 when we are three dimensional? Wouldn’t kg/m^3 or g/cm^3 be more accurate?
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It’s infuriating that nobody’s answering your question.
Everyone’s just saying “because that’s the way it is”.
The true answer is: you’re right.
kg/m^3 (called the CI, or Corpulence Index) is an alternative to BMI which is known to be just flat-out superior.
BMI is fundamentally flawed, for exactly the reason (among others) that you have mentioned.
That is known information.
Anyone saying BMI just coincidentally happens to describe someone’s thinness accurately is misinformed: kg/m^3 is more accurate.
BMI was popularized at a time when nobody really cared about these things and there weren’t any popular alternatives, so it got established in the 1970s as just what people used.
Since then it’s been kind of caught in a feedback loop, where because it was popular, people published things about it, and because people published things about it, it got more popular.
In the grand scheme of things, whether you use kg/m^2 or kg/m^3 (or kg/m^2.5 ) doesn’t matter *that* much.
BMI is good enough at average heights, and only becomes problematically inaccurate when people are very short or very tall.
(Basically, if you’re very tall, you will always be misreported as being overweight by BMI when you’re actually at a healthy weight)
But…most people are of an average height, we mostly only care about BMI as a population statistic anyway (where we’re focused on the average person), so it continues to persist as fundamentally flawed, but, meh, good enough most of the time.
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