BMI at age 12
Adult BMI bands do not apply at age 12. The form below gives a percentile for age and sex, and the example walks through one case.
How it works
BMI is weight in kilograms divided by height in meters squared. For a child that number is scored against the CDC table. M is the median BMI for the age and sex. L and S set the shape of the spread. The z score and percentile follow from all three. The table covers ages 2 to 20, and this male page sits at age 12.
Worked example
Say a boy aged 12 is 4 ft 11 in tall and weighs 88 lb. That is 1.4986 m and 39.916 kg. Square the height: 1.4986 × 1.4986 = 2.2458. Divide 39.916 by 2.2458 to get 17.7736. The table gives L -2.4474, M 17.815 and S 0.13139. Divide 17.7736 by 17.815 to get 0.997676. 0.997676 to the power of -2.4474 is 1.005711. Subtract 1 to get 0.005711. Divide by (-2.4474 × 0.13139) to get z = -0.018. That is the 49th percentile, which is healthy weight.
At age 12, 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 (4 ft 11 in, 88 lb)
- BMI 17.8, 49th percentile
- Female at median height and weight (5 ft 0 in, 93 lb)
- BMI 18.2, 51st percentile
Common questions
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 17.8 at age 12.
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 12 is 21.0.
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 144.5 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 12 the median BMI is 17.8 for males and 18.1 for females, and the example boy sits at the 49th 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 12 the 5th, 85th and 95th percentile BMI values for males are 15.0, 21.0 and 24.2.