A BMI (Body Mass Index) calculator is a tool used to assess an individual's body weight relative to their height. It's a widely used metric for categorizing individuals into different weight categories, such as underweight, normal weight, overweight, and obese. The BMI is calculated using the following formula:

BMI = weight (kg) / (height (m) * height (m))

Where weight is measured in kilograms and height is measured in meters.

Once the BMI value is calculated, it is compared to standard ranges to determine the individual's weight category:

BMI below 16: Very Severely Underweight
BMI 16 to 16.9: Severely Underweight
BMI 17 to 18.4: Underweight
BMI 18.5 to 24.9: Normal weight
BMI 25 to 29.9: Overweight
BMI 30 to 34.9: Obese Class I
BMI 35 to 39.9: Obese Class II
BMI 40 or higher: Obese Class III

It's important to note that while BMI is a useful tool for population-level assessments of weight status, it does not directly measure body fat percentage or distribution. Therefore, it may not accurately reflect the health status of individuals with high muscle mass or other factors that can influence weight. It's best used as part of a comprehensive assessment of an individual's health and lifestyle factors.