BMI at age 20
Adult BMI bands do not apply at age 20. The form below gives a percentile for age and sex, and the example walks through one case.
How it works
BMI is weight in kilograms divided by height in meters squared. For a child that number is scored against the CDC table. M is the median BMI for the age and sex. L and S set the shape of the spread. The z score and percentile follow from all three. This page reads the CDC male table at 240.5 months.
Worked example
Say a boy aged 20 is 5 ft 10 in tall and weighs 161 lb. That is 1.7780 m and 73.028 kg. Square the height: 1.7780 × 1.7780 = 3.1613. Divide 73.028 by 3.1613 to get 23.1006. The table gives L -1.8436, M 23.041 and S 0.13468. Divide 23.1006 by 23.041 to get 1.002587. 1.002587 to the power of -1.8436 is 0.995248. Subtract 1 to get -0.004752. Divide by (-1.8436 × 0.13468) to get z = 0.019. That is the 51st percentile, which is healthy weight.
At age 20, 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 (5 ft 10 in, 161 lb)
- BMI 23.1, 51st percentile
- Female at median height and weight (5 ft 4 in, 127 lb)
- BMI 21.8, 51st 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 23.1 for this boy at age 20 says nothing about how much of the 161 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 23.0 at age 20.
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 20 is 27.1.
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 240.5 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 20 the median BMI is 23.0 for males and 21.7 for females, and the example boy sits at the 51st percentile.