BMI at age 8
At 8 years old a BMI number only means something next to other children the same age. Use the form for your child, or read the worked example below.
How it works
First the page works out BMI: weight in kilograms divided by height in meters squared. Then it reads L, M and S from the CDC table for this age and sex. It finds z as ((BMI divided by M) to the power L minus 1) divided by (L times S) and turns z into a percentile. This page reads the CDC male table at 96.5 months.
Worked example
Convert first: 4 ft 2 in is 127 cm and 56 lb is 25.401 kg. Square the height: 1.2700 × 1.2700 = 1.6129. Divide 25.401 by 1.6129 to get 15.7487. Look up the male row for age 8: L -3.1831, M 15.782 and S 0.10209. Divide 15.7487 by 15.782 to get 0.997890. Raise that to the power -3.1831 to get 1.006746. Subtract 1 to get 0.006746. Divide by -3.1831 × 0.10209 to get z = -0.021. That is the 49th percentile, in the healthy weight band.
At age 8, 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 2 in, 56 lb)
- BMI 15.7, 49th percentile
- Female at median height and weight (4 ft 2 in, 56 lb)
- BMI 15.7, 48th percentile
Common questions
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 15.7 for this boy at age 8 says nothing about how much of the 56 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 15.8 at age 8.
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 8 is 18.0.
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 96.5 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 8 the median BMI is 15.8 for males and 15.8 for females, and the example boy sits at the 49th percentile.