Medical professionals use body mass index, or BMI, to gauge the amount of a person's body fat. It’s a simple, inexpensive method in comparison to underwater weighing or the caliper test. An adult’s BMI is calculated by plugging the person's height and weight into a standard formula. The resulting number is called a BMI score. Scores indicate whether a person is underweight, normal, overweight or obese. Because children and teenagers from ages 2 to 20 are still growing, their BMI scores are compared to others of the same age and gender on gender-specific BMI-for-age percentile charts, according to the Centers for Disease Control and Prevention. Children’s percentiles categorize them as being either underweight, a healthy weight, overweight or obese.
Step 1
Measure your child’s height by having her stand barefooted and with good posture against the wall. Make a light pencil mark on the wall at the height of the crown of the head. Use a measuring tape to measure the distance from the mark to the floor, in inches. Record the number.
Step 2
Weigh your child, in pounds, on a bathroom scale. If you have a scale that reads weight to the nearest tenth of a percent, record the figure without rounding it up or down.
Step 3
Use the BMI formula to calculate your child’s raw BMI score. Divide the child's weight by height squared, and multiply the resulting number by 703, which is a conversion factor for the metric system: BMI score = weight in pounds / height in inches x height in inches x 703. If your child is 48 inches tall and weighs 67.5 pounds, the BMI score would be 20.6 with this formula.
Step 4
Use a BMI-for-age percentile chart that corresponds to your child’s gender. If you have a daughter, trace your finger from her age on the bottom of the girl’s chart to her BMI score. A 12-year-old girl with a BMI score of 20.6 is just above the 75th percentile, whereas a 15-year-old girl with the same score is just above the 50th percentile. Use the boy's chart if you have a son. A 12-year-old boy with a BMI score of 20.6 is also just above the 75th percentile, and a 15-year-old boy with the same score is just under the 75th percentile line.
Step 5
Interpret the chart. For a 12-year-old girl with a BMI score of 20.6, who is just above the 75th percentile, 75 percent of girls her age have the same or less body fat, while 25 percent have more.
Percentile ranges are also categorized. A child who scores below the fifth percentile is underweight, one who scores between the fifth and 85th percentiles is within a normal weight range, while a child who scores between the 85th and 95th percentile is overweight. Scoring above the 95th percentile is considered obese.
Tips and Warnings
- The calculation for an adult BMI score is the same as it is for children: weight in pounds / height in inches x height in inches x 703 = BMI score. Adult BMI calculations do not factor in age, nor are the resulting scores compared to those of the same age or gender, notes the CDC. Any adult, male or female, ages 21 and over, who has a BMI score under 18.5 is considered underweight. Likewise, adults scoring between 18.5 and 24.9 are considered to be at a healthy weight, while those scoring between 25 and 29.9 are considered overweight. Scoring 30 and above classifies an adult as obese.
Things You'll Need
- Bathroom scale
- Tape measure
- BMI chart
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