No magic number exists as the ideal weight for all teens because no two people are shaped exactly alike. Some teens are tall and lanky while others are short and muscular. By plugging in weight and height, a teen can use a body mass index calculator to get a good idea if she has a healthy weight or if she has a weight problem that could lead to health complications. However, BMI does not differentiate between weight from muscle and weight from fat.
Understanding BMI
A teen's BMI does not measure fat directly, but it generally gives a good reading that is comparable to direct readings such as from underwater weighing. A teen's BMI is age- and sex-specific. Body fat changes with age, especially as teens go through puberty. Once plotted on the Centers for Disease Control and Prevention BMI-for-age growth charts for either male or female, a number gets a percentile rating that shows how the teen compares with other teens of the same height, age and sex. That percentile will correspond to one of four categories to define weight: underweight, healthy weight, overweight and obese.
Reading BMI
A normal BMI for a teen falls between the fifth and 84th percentiles. A BMI below the fifth percentile is underweight and may signal an eating disorder. BMI between the 85th and 94th percentiles is overweight; and a BMI in the 95th percentile or higher is obese. Being overweight puts a teen at risk for health problems, including Type 2 diabetes, high blood pressure, high cholesterol, coronary heart disease and cancers. Because BMI is not a direct reading of body fat, it does not always give an accurate result. BMI does not take into account a teen's body frame. A muscular teen may not be overweight, but can fall into the overweight category on a BMI-for-age and sex growth chart because of muscle weight. Likewise, a petite teen with excess fat could land in a healthy weight on the chart.
Calculating BMI
The BMI formula for teens is the same as that for adults. To calculate BMI with the metric system, divide weight in kilograms by height in meters squared. If height is calculated in centimeters and not meters, divide the weight in kilograms by the height in centimeters squared and then multiply the result by 10,000. For the English measurement formula, convert ounces and fractions to decimal values. Then, divide weight in pounds by height in inches squared and multiply by a conversion factor of 703.
Using Online Calculators
You can find BMI calculators for teens online. An appropriate teen calculator asks gender, birth date and height in inches and weight in pounds, or asks for height in centimeters and weight in kilograms. The CDC uses the same calculator for ages 2 to 20. The CDC offers a free BMI calculator for teens under its Healthy Weight section. KidsHealth from The Nemours Foundation also offers a free BMI calculator for teens.
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