Why can't I just use an adult BMI category for a child?

Children's healthy BMI changes dramatically with age and differs by sex, so a fixed cutoff like 25 makes no sense. Instead a child's BMI is ranked against a reference population of the same age and sex, producing a percentile that accounts for normal growth.

What is the LMS method?

The CDC growth charts store three numbers per age and sex: L (a power that corrects skewness), M (the median BMI), and S (the spread). A child's BMI is converted to a z-score with these, and the z-score becomes a percentile via the normal distribution.

What do the CDC categories mean?

Below the 5th percentile is underweight, the 5th up to the 85th is a healthy weight, the 85th up to the 95th is overweight, and the 95th percentile or above is obesity. These bands are the standard CDC definitions for children and teens aged 2 to 20.

Why is this only for ages 2 to 20?

The CDC BMI-for-age reference covers ages 2 through 20 years. For infants and toddlers under 2, clinicians use the WHO weight-for-length chart instead, which is a different reference and not interchangeable with BMI percentiles.

How precise is the percentile?

The tool interpolates the published CDC LMS parameters to the child's exact age, so the result is accurate for screening. At very high BMI the CDC applies a modified extreme z-score that this general calculator does not, so the most extreme values are approximate.

Pediatric BMI Percentile Calculator (CDC)

A child’s body mass index cannot be read against a single fixed cutoff the way an adult’s can, because healthy BMI rises and falls naturally through childhood and differs between boys and girls. Paediatric assessment instead asks where a child sits relative to peers of the same age and sex, expressed as a percentile.

How it works

First the ordinary BMI, then a transformation to a z-score using the CDC LMS parameters for the child’s age and sex:

BMI = weight(kg) / height(m)^2

if L != 0:  z = ((BMI / M)^L - 1) / (L x S)
if L == 0:  z = ln(BMI / M) / S

percentile = normalCDF(z) x 100

Here L corrects the skewed shape of the BMI distribution, M is the median BMI for that age and sex, and S is the coefficient of variation. The tool stores yearly CDC LMS rows from age 2 to 20 and interpolates them to the child’s exact age in months.

Example and notes

An 8-year-old boy who is 128 cm and 26 kg has a BMI of about 15.9. Compared with the CDC reference for boys aged 8, that lands near the 60th to 70th percentile, a healthy weight. Use whole years plus months for the most accurate result, and remember that for children under 2 the WHO weight-for-length chart applies instead of BMI-for-age.