With obesity and weight problems continually climbing, it's important to evaluate children for healthy body mass index before it becomes a problem. While they're only a starting point, BMI calculations are a simple way to determine whether your child might be in danger of obesity or excess body fat. There are several variations in what makes a healthy level of body fat in kids, so BMI calculations for anyone between 2 and 20 are very specific and must follow a certain procedure.
Definition
BMI, or Body Mass Index, is a number that's calculated from a person's weight and height. It doesn't directly measure body fat, but is a good alternative for it because it correlates to direct measures of body fat, such as underwater weighing. It's also an inexpensive and easy way to screen for weight categories that could lead to health problems. For most children and teens, it reliably indicates their body fatness, and is calculated specific to age and gender. This is called BMI-for-age.
Percentile
After a child's BMI is calculated, it's plotted on a BMI-for-age growth chart, specific to either boys or girls, to get a percentile ranking. The percentile shows where the child fits in among other kids of the same age and gender, and what category he fits under: underweight, healthy weight, overweight or obese.
If a child is less than the fifth percentile, or his BMI is only greater than 5 percent of kids his age, he's underweight. Healthy weight falls between the fifth and 85th percentiles. If a child is in the 85th to 95th percentile, meaning he weighs more than 85 to 95 percent of children the same age, he's overweight, and at the 95th percentile or above, he's considered obese.
Assessment
BMI is used to screen and identify possible or potential weight problems in children. It's recommended that BMI screening begin at 2 years old.
However, the numbers don't tell the whole story. Because BMI doesn't directly measure body fat, a child in the 85th to 95th percentile might not have excess fat. It's possible that he is very athletic and has a high muscle mass. A health care provider would need to do further tests and assessments to figure out whether too much body fat is an issue. That could include skinfold thickness measurements and evaluation of the child's diet, level of physical activity and family history.
Calculation
Before calculating a child's BMI, you need his birth date, height and weight measurements. The height and weight are very specific---you'll input height to the nearest 1/8" and weight to the nearest ¼ pound. Then use the Child and Teen BMI Percentile Calculator (see Resources), to figure out his BMI.
Results
Your results are given based on the BMI-for-age percentile, which is different than calculating BMI for an adult. The reason age and gender are considered for children and teens is that the amount of body fat changes with age, and it's different between boys and girls. A BMI of 20, for instance, will be obese for a 6-year-old boy, overweight for a 9-year-old boy and healthy weight for a 14-year-old boy.
The calculated BMI-for-age percentile will tell you whether your child is underweight, healthy weight, overweight or obese. Again, below the fifth percentile is underweight, between the 85th and 95th is overweight, and 95th or above is obese; anything in between is a healthy weight. There are no guidelines for healthy weight ranges, because they change so much with height, gender, and by each month your child gets older.
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