BMI is the standard. Your doctor calculates it. The Department of Health publishes it. The category labels (underweight, healthy, overweight, obese) are on every public health poster. It tells you whether you're fit, whether you're at risk, what insurance bracket you're in, whether you should be losing weight. Plug height and weight, get verdict. Easy. Universal. Scientific.
BMI was invented by Belgian mathematician Adolphe Quetelet in 1832 as a population-statistics tool, the 'average man' calculation, never intended as a diagnostic for individuals. Insurance companies adopted it in the 1970s because it was cheap and computable. The modern category cutoffs were finalised by a 1998 NIH committee that, controversially, lowered the 'overweight' threshold and reclassified about 30 million Americans as overweight overnight. BMI does not distinguish between muscle and fat, between fat distribution patterns, between bone density. Most rugby props are 'obese' by BMI. Most champion gymnasts are 'underweight.' At a population level it correlates loosely with health outcomes; at an individual level, particularly for non-European body compositions and for muscular people, it is a famously bad measure. The American Medical Association formally cautioned against using it as a sole diagnostic in 2023. Waist-to-height ratio, body fat percentage, blood markers, fitness tests, all do better. BMI is a 19th-century proxy used because it's quick, not because it's right.