BMI at age 4
This calculator places a BMI on the CDC growth chart for age 4. The sample is a boy at 3 ft 4 in and 36 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. For males at age 4 the 5th, 85th and 95th percentile BMI values are 14.0, 16.9 and 17.8.
Worked example
For a boy aged 4 at 3 ft 4 in and 36 lb, height is 1.0160 m and weight is 16.329 kg. Square the height: 1.0160 × 1.0160 = 1.0323. BMI is 16.329 divided by 1.0323, which is 15.8181. The CDC values are L -1.7149, M 15.628 and S 0.07189. Divide the BMI by M to get 1.012164. Raise that to the power L to get 0.979479. Subtract 1 to get -0.020521. Divide by L × S to get z = 0.166. The 57th percentile falls in the healthy weight band.
At age 4, 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 4 in, 36 lb)
- BMI 15.8, 57th percentile
- Female at median height and weight (3 ft 4 in, 35 lb)
- BMI 15.4, 53rd percentile
Common questions
What is the median BMI at age 4?
On the CDC chart the median is 15.6 for males and 15.3 for females at age 4. The example BMI of 15.8 is 0.2 away from the male median of 15.6.
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 15.8 for this boy at age 4 says nothing about how much of the 36 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 15.6 at age 4.
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 4 is 16.9.
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 48.5 months.