BMI at age 18
Enter an age, a sex, a height and a weight to see BMI for age. The example below uses a boy aged 18 at 5 ft 9 in and 148 lb.
How it works
BMI is weight in kilograms divided by height in meters squared. The page then looks up three CDC values for the age and sex: L, M and S. The z score is ((BMI divided by M) to the power L minus 1) divided by (L times S). The percentile comes from the z score. This page reads the CDC male table at 216.5 months.
Worked example
Take a boy aged 18 who is 5 ft 9 in tall and weighs 148 lb. In metric that is 175.26 cm and 67.132 kg. Square the height: 1.7526 × 1.7526 = 3.0716. Divide 67.132 by 3.0716 to get a BMI of 21.8557. The CDC table for males at age 18 gives L -1.8747, M 21.896 and S 0.13229. Divide the BMI by M: 21.8557 divided by 21.896 = 0.998159. Raise that to the power L to get 1.003460. Subtract 1 to get 0.003460. Divide by L × S: 0.003460 divided by (-1.8747 × 0.13229) = -0.014. A z score of -0.014 is the 49th percentile, which is in the healthy weight band.
At age 18, 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 9 in, 148 lb)
- BMI 21.9, 49th percentile
- Female at median height and weight (5 ft 4 in, 124 lb)
- BMI 21.3, 50th percentile
Common questions
What is the median BMI at age 18?
On the CDC chart the median is 21.9 for males and 21.3 for females at age 18. The example BMI of 21.9 is 0.0 away from the male median of 21.9.
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 21.9 for this boy at age 18 says nothing about how much of the 148 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 21.9 at age 18.
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 18 is 25.7.
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 216.5 months.