BMI (Body Mass Index) is a simple number doctors get from your height and weight. It's a rough way to sort people into ranges: under 25 is 'normal,' 25–30 is 'overweight,' and 30 or higher is 'obese.' It isn't perfect — a very muscular person can have a high BMI without much fat — but it's the number most weight-loss guidelines use to decide who a medicine is meant for.