BMI at age 3
Adult BMI bands do not apply at age 3. The form below gives a percentile for age and sex, and the example walks through one case.
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. This page reads the CDC male table at 36.5 months.
Worked example
Say a boy aged 3 is 3 ft 2 in tall and weighs 33 lb. That is 0.9652 m and 14.969 kg. Square the height: 0.9652 × 0.9652 = 0.9316. Divide 14.969 by 0.9316 to get 16.0681. The table gives L -1.4200, M 16.000 and S 0.07263. Divide 16.0681 by 16.000 to get 1.004256. 1.004256 to the power of -1.4200 is 0.993987. Subtract 1 to get -0.006013. Divide by (-1.4200 × 0.07263) to get z = 0.058. That is the 52nd percentile, which is healthy weight.
At age 3, 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 (3 ft 2 in, 33 lb)
- BMI 16.1, 52nd percentile
- Female at median height and weight (3 ft 1 in, 31 lb)
- BMI 15.9, 57th percentile
Common questions
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 3 the median BMI is 16.0 for males and 15.7 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 3 the 5th, 85th and 95th percentile BMI values for males are 14.3, 17.3 and 18.2.
What is the median BMI at age 3?
On the CDC chart the median is 16.0 for males and 15.7 for females at age 3. The example BMI of 16.1 is 0.1 away from the male median of 16.0.
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 16.1 for this boy at age 3 says nothing about how much of the 33 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 16.0 at age 3.