Body Mass Index (BMI) is calculated by dividing a person's weight in kilograms by their height in meters squared. The formula is BMI = weight (kg) / height (m²). For instance, a person weighing 70 kg and measuring 1.75 meters tall would have a BMI of approximately 22.9. BMI is a useful screening tool for assessing body fat and categorizing individuals into underweight, normal weight, overweight, or obese classifications, helping guide health decisions.