Body Mass Index compares your weight to your height to give a quick, population-level screen for weight status. It's the first number most doctors glance at, not because it's perfect, but because it's fast, free, and correlates reasonably well with health risk across large groups of people.
Ranges: under 18.5 underweight · 18.5–24.9 healthy · 25–29.9 overweight · 30+ obese.
BMI is weight in kilograms divided by height in metres squared. The result sorts into four broad bands: under 18.5 is considered underweight, 18.5 to 24.9 is the healthy range, 25 to 29.9 is overweight, and 30 and above is classed as obese. It's a useful screening number, but it can't tell the difference between muscle and fat, so athletes and very muscular people often score higher than their actual body composition would suggest.