BMI at age 9
Enter an age, a sex, a height and a weight to see BMI for age. The example below uses a boy aged 9 at 4 ft 5 in and 65 lb.
How it works
BMI is weight in kilograms divided by height in meters squared. The page then looks up three CDC values for the age and sex: L, M and S. The z score is ((BMI divided by M) to the power L minus 1) divided by (L times S). The percentile comes from the z score. For males at age 9 the 5th, 85th and 95th percentile BMI values are 14.0, 18.6 and 21.1.
Worked example
Take a boy aged 9 who is 4 ft 5 in tall and weighs 65 lb. In metric that is 134.62 cm and 29.484 kg. Square the height: 1.3462 × 1.3462 = 1.8123. Divide 29.484 by 1.8123 to get a BMI of 16.2688. The CDC table for males at age 9 gives L -2.9711, M 16.167 and S 0.11172. Divide the BMI by M: 16.2688 divided by 16.167 = 1.006297. Raise that to the power L to get 0.981522. Subtract 1 to get -0.018478. Divide by L × S: -0.018478 divided by (-2.9711 × 0.11172) = 0.056. A z score of 0.056 is the 52nd percentile, which is in the healthy weight band.
At age 9, 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 5 in, 65 lb)
- BMI 16.3, 52nd percentile
- Female at median height and weight (4 ft 4 in, 63 lb)
- BMI 16.4, 51st percentile
Common questions
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 9 is 18.6.
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 108.5 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 9 the median BMI is 16.2 for males and 16.3 for females, and the example boy sits at the 52nd 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 9 the 5th, 85th and 95th percentile BMI values for males are 14.0, 18.6 and 21.1.
What is the median BMI at age 9?
On the CDC chart the median is 16.2 for males and 16.3 for females at age 9. The example BMI of 16.3 is 0.1 away from the male median of 16.2.