BMI is body mass index: a simple number from your weight and height, used as a rough map of whether a person is underweight, in a middle band, overweight, or obese on a chart. The usual sum is weight in kilograms divided by height in metres squared. Clinics, gyms, and school health checks love it because it is quick. The tidy story is that certain bands mean healthy; the messier truth is that BMI cannot see the difference between muscle and fat, and it was built on old averages, so a stocky athlete can look overweight on paper, and two people with the same BMI can be in very different shape. Some East Asian guidelines use slightly lower cut-offs than older Western charts, because risk can show up at a smaller number. People stretch the letters in conversation — What’s your BMI? — as if it were a full health report. It is only one clue. Said as letters: B-M-I.