Body mass index (BMI) is a numerical value calculated based on a person's weight and height. It is commonly used as an indicator of healthy weight and is often used to classify individuals as underweight, normal weight, overweight, or obese. BMI is calculated by dividing an individual's weight in kilograms by the square of their height in meters. A BMI of 18.5-24.9 is considered normal, while a BMI below 18.5 is considered underweight and a BMI above 25 is considered overweight or obese. BMI is a useful tool for assessing overall health and risk for various chronic diseases, but it is important to consider other factors such as muscle mass and body composition when interpreting BMI results.