A lean, heavily muscled athlete can score "overweight" or even "obese" on BMI despite having very low body fat — the formula only ever sees total mass and height, never composition.
BMI was built by a 19th-century mathematician studying population averages, not individual bodies, using nothing more than mass divided by height squared. It correlates reasonably well with body fat across a whole population, but for any one person carrying unusual amounts of muscle, it simply has no way to know.
Estimate with the rule, then check it against the calculator that models it properly.
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