BMI at age 15
Children and teens are compared with others of the same age and sex. BMI is read as a percentile. This page shows how that works at age 15.
How it works
The page divides weight in kilograms by height in meters squared, then compares the result with the CDC table for the same age and sex. It uses the table's L, M and S values to get a z score. Then it converts that to a percentile. For males at age 15 the median is M 19.858.
Worked example
Say a boy aged 15 is 5 ft 7 in tall and weighs 127 lb. That is 1.7018 m and 57.606 kg. Square the height: 1.7018 × 1.7018 = 2.8961. Divide 57.606 by 2.8961 to get 19.8909. The table gives L -2.1323, M 19.858 and S 0.13511. Divide 19.8909 by 19.858 to get 1.001657. 1.001657 to the power of -2.1323 is 0.996476. Subtract 1 to get -0.003524. Divide by (-2.1323 × 0.13511) to get z = 0.012. That is the 50th percentile, which is healthy weight.
At age 15, a boy and a girl each at the CDC median height and the weight that gives the median BMI. The form above is set to the boy.
- Male at median height and weight (5 ft 7 in, 127 lb)
- BMI 19.9, 50th percentile
- Female at median height and weight (5 ft 4 in, 116 lb)
- BMI 19.9, 50th percentile
Common questions
Does BMI show body fat in a child?
Only roughly. BMI uses height and weight. A muscular child can score high. A doctor looks at growth over time as well as the number. A BMI of 19.9 for this boy at age 15 says nothing about how much of the 127 lb is muscle.
Why does BMI change as a child grows?
BMI usually falls through early childhood and then rises through the teen years. That is normal growth, which is why the chart is built by age. The median BMI for males is 19.9 at age 15.
What should I do if my child is above the 85th percentile?
One reading is not a diagnosis. Raise it with your child's doctor, who can look at growth history and the whole picture. The 85th percentile BMI for males at age 15 is 23.5.
Which ages does this calculator cover?
It covers ages 2 to 20, the range of the CDC growth charts. For adults over 20 use the adult BMI calculator. This page reads the CDC male table at 180.5 months.
Why does a child BMI use percentiles and not the adult bands?
Children grow, and a healthy BMI changes with age and differs between males and females. A percentile shows where a child sits among others of the same age and sex. At age 15 the median BMI is 19.9 for males and 19.9 for females, and the example boy sits at the 50th percentile.