BMI is weight in kilograms divided by height in metres squared. It was designed in the 1830s as a population-level screen, not a clinical measure of any individual's health. The standard adult categories are: under 18.5 underweight, 18.5 to 24.9 healthy range, 25.0 to 29.9 overweight, 30 and above obese.
Where BMI breaks
BMI cannot tell muscle from fat. A 95 kg rugby player at 12% body fat lands in the obese range. A sedentary office worker at the same height and 25% body fat can score within the healthy range despite carrying more fat mass. BMI also runs slightly high for people of African descent and slightly low for people of South and East Asian descent at the same level of metabolic risk.
What to pair it with
Waist circumference is a stronger single predictor of metabolic risk than BMI. Aim under 94 cm for men and under 80 cm for women, or under half your height in cm regardless of sex. Add a monthly progress photo in the same lighting and a body-fat estimate from a DEXA, BIA scale or skinfold caliper. Use BMI as the cheapest first check, then trust the other three over it when they disagree.
