BMI at age 13
This calculator places a BMI on the CDC growth chart for age 13. The sample is a boy at 5 ft 2 in and 101 lb, with a girl of the same age in the table.
How it works
There are three steps. Work out BMI. Read L, M and S from the CDC table for the age and sex. Then find z with ((BMI divided by M) to the power L minus 1) divided by (L times S) and convert z to a percentile. This page reads the CDC male table at 156.5 months.
Worked example
Convert first: 5 ft 2 in is 157.48 cm and 101 lb is 45.813 kg. Square the height: 1.5748 × 1.5748 = 2.4800. Divide 45.813 by 2.4800 to get 18.4730. Look up the male row for age 13: L -2.3295, M 18.472 and S 0.13414. Divide 18.4730 by 18.472 to get 1.000054. Raise that to the power -2.3295 to get 0.999874. Subtract 1 to get -0.000126. Divide by -2.3295 × 0.13414 to get z = 0.000. That is the 50th percentile, in the healthy weight band.
At age 13, 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 2 in, 101 lb)
- BMI 18.5, 50th percentile
- Female at median height and weight (5 ft 2 in, 102 lb)
- BMI 18.7, 49th percentile
Common questions
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 156.5 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 13 the median BMI is 18.5 for males and 18.7 for females, and the example boy sits at the 50th percentile.
What do the percentile bands mean?
Below the 5th percentile is underweight. From the 5th up to the 85th is a healthy weight. From the 85th up to the 95th is overweight. The 95th and above is obesity. At age 13 the 5th, 85th and 95th percentile BMI values for males are 15.5, 21.9 and 25.2.
What is the median BMI at age 13?
On the CDC chart the median is 18.5 for males and 18.7 for females at age 13. The example BMI of 18.5 is 0.0 away from the male median of 18.5.
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 18.5 for this boy at age 13 says nothing about how much of the 101 lb is muscle.