BMI at age 6
Children and teens are compared with others of the same age and sex. BMI is read as a percentile. This page shows how that works at age 6.
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. At age 6 the median BMI is 15.4 for males and 15.2 for females. This male example is 0.1 from its median.
Worked example
Take a boy aged 6 who is 3 ft 10 in tall and weighs 46 lb. In metric that is 116.84 cm and 20.865 kg. Square the height: 1.1684 × 1.1684 = 1.3652. Divide 20.865 by 1.3652 to get a BMI of 15.2835. The CDC table for males at age 6 gives L -3.2117, M 15.384 and S 0.08305. Divide the BMI by M: 15.2835 divided by 15.384 = 0.993467. Raise that to the power L to get 1.021274. Subtract 1 to get 0.021274. Divide by L × S: 0.021274 divided by (-3.2117 × 0.08305) = -0.080. A z score of -0.080 is the 47th percentile, which is in the healthy weight band.
At age 6, 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 10 in, 46 lb)
- BMI 15.3, 47th percentile
- Female at median height and weight (3 ft 9 in, 44 lb)
- BMI 15.3, 52nd 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 72.5 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 6 the median BMI is 15.4 for males and 15.2 for females, and the example boy sits at the 47th 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 6 the 5th, 85th and 95th percentile BMI values for males are 13.7, 17.0 and 18.4.
What is the median BMI at age 6?
On the CDC chart the median is 15.4 for males and 15.2 for females at age 6. The example BMI of 15.3 is 0.1 away from the male median of 15.4.
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 15.3 for this boy at age 6 says nothing about how much of the 46 lb is muscle.