Body Mass Index is your weight in kilograms divided by the square of your height in metres: BMI = kg ÷ m². Imperial values are converted first (1 lb = 0.4536 kg, 1 in = 2.54 cm).
These are the World Health Organization adult categories. They don’t apply to children, and some populations use lower cut-offs.
BMI doesn’t distinguish muscle from fat, so athletes can score “overweight” while being lean. BMI Prime (BMI ÷ 25; aim for 1.0 or below) shows how far you are from the upper healthy limit; a waist measurement is a useful second check. For medical advice, talk to a healthcare professional.