Body Mass Index (BMI) is a calculation of the level of fat in a person’s body. Typically, BMI is used as an estimation of body fat, however, rather than an exact measurement. This number is based on an individual’s weight-to-height ratio. The formula for calculating BMI is basically the same for everyone, but the number that constitutes a healthy BMI may vary a bit when it comes to children and teens. Essentially, however, a healthy BMI number is low enough that a person isn’t at risk of developing an obesity-related health condition but not so low that he’s at risk for health problems related to being underweight.
Calculating one's BMI is usually pretty simple. To do this, a person starts by figuring out what his weight is in kilograms. He then figures out what his height is in meters squared. He can then divide his weight by his height in order to come up with his BMI.
A person may use an online BMI chart to see where he stands when it comes to body fat. This type of chart will usually reveal that a healthy BMI is between 18.5 and 24.9. If a person has a BMI of 25 or above, he is usually considered overweight, and a BMI of 30 or above means the person is considered obese. If an individual has a BMI of 18.5 or lower, however, that person is usually considered underweight.
Determining what constitutes a healthy BMI for a child or teen differs from evaluating the BMI of a person who is at least 20 years old. There is a separate BMI chart that is used for people who are younger than 20. This chart considers the individual’s age and gender, as the teen or child usually has more growth ahead of him, and puberty can change what counts as a healthy BMI. Besides using a chart designed specifically for children and teens, a person may ask a doctor for healthy BMI ranges for people in this age category.
It's important to note that a person who has a high BMI may not always have too much body fat or be considered overweight or obese. Some athletes have an increased amount of muscle that contributes to higher numbers in BMI calculations. The increased muscle often skews the calculations despite the fact that the athlete has a healthy amount of body fat.