Body Mass Index (BMI) is a numerical value calculated from a person's weight and height. It is used as a simple method to assess whether an individual has a healthy body weight for their height. The formula for BMI is weight in kilograms divided by height in meters squared. BMI categories include underweight, normal weight, overweight, and obesity. While it is a useful screening tool, BMI does not directly measure body fat or account for muscle mass, age, or gender. Therefore, it should be used alongside other assessments for a more comprehensive understanding of health.