BMI stands for Body Mass Index, which is a measurement of body fat based on a person's weight and height. It is a commonly used tool to assess if a person is underweight, overweight or within a healthy weight range.

The formula to calculate BMI is: weight (kg) / (height (m))^2.

A BMI below 18.5 is considered underweight, a BMI between 18.5 and 24.9 is considered normal weight in both men and women. A BMI between 25 and 29.9 is considered overweight, and a BMI of 30 or higher is considered obese.

For teenagers, the healthy weight range for a teenager can vary depending on their age, gender, height, and overall health status. A useful tool to determine a healthy weight range for a teenager is the BMI (Body Mass Index) chart for their age and gender.

In general, a healthy BMI range for teenagers aged 13-19 falls between the 5th and 85th percentile, although the ideal range may vary based on individual circumstances.

For both men and women, a BMI between 18.5 and 24.9 is considered normal weight. A high BMI (over 25) can indicate an increased risk for a number of health problems, including high blood pressure, diabetes, heart disease, stroke, and certain cancers. A low BMI (below 18.5) can indicate malnutrition or an increased risk for certain health problems, such as osteoporosis.

While BMI is not a perfect measurement of body fat and does not account for individual differences in body composition, it can be a useful tool for identifying potential health risks associated with weight. However, it is important to note that a healthy BMI range does not guarantee good health, and other factors such as physical activity, diet, and overall health status should also be taken into account when assessing overall health.