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> BMI is a pretty decent measure of fat percentage for the vast majority of people

It isn't really measuring anything. It is just a very general index to gauge how far you diverge from a predetermined height to weight ratio.

Maybe that's useful to someone but I wouldn't base any of my decisions on it today.

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On the margins it may not be useful, but for the "average" person, it is definitely useful. I'd suggest there is no one with a BMI of 30 (besides people with a ton of muscle) that is at a healthy weight. Its a gut check, which is what is helpful.

You know what they say, most of the volume of a high-dimensional orange is in the skin. The average person is very rare.

BMI is best for populations since it's so easy to calculate from easily and frequently collected data. For an individual, you can fairly easily get more accurate measures to predict whatever it is you want to predict.

If you're not a bodybuilder and not pregnant, it's one of the simplest useful calculations we have. You'd be amazed at how many medical decision-making tools[0] are very simple and are based on outcomes, not rigorous mathematical theory.

[0] E.g. NEWS https://www.nice.org.uk/advice/mib205/chapter/The-technology

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