About the Child BMI Percentile Calculator
This child BMI percentile calculator works out a child’s or teen’s body mass index and then places it on the CDC BMI-for-age growth chart for their sex and exact age. For anyone aged 2 to 19, BMI alone is not meaningful because body fat changes a lot as children grow — the percentile is what tells you how a child compares with others of the same age and sex.
Parents, school nurses, coaches and health students can use it to understand a measurement from a checkup or to track a child over time. It returns the percentile, the CDC weight status category (underweight, healthy weight, overweight or obesity), the z-score, the BMI at the 95th percentile and a healthy weight range for the child’s current height.
The calculation uses the official CDC 2000 LMS parameters for every half-month of age from 2 to 20 years. A percentile is a screening measure, not a diagnosis: a child who is very muscular or going through puberty early or late can fall outside the healthy range while being perfectly healthy. Talk to your pediatrician before making changes to a child’s diet or activity.
With the default inputs, the bmi-for-age percentile is 54.5 th percentile. Change any value above to recalculate instantly.
How to use the child bmi percentile calculator
- 1Choose the child’s sex — boys and girls have separate CDC charts.
- 2Enter age in whole years plus any extra months (ages 2 to 19).
- 3Pick your units and enter the child’s weight and height without shoes.
- 4Read the BMI-for-age percentile and CDC weight status category.
- 5Use the healthy weight range and chart as talking points with your pediatrician.
Formula and method
First the calculator computes BMI as weight in kilograms divided by height in metres squared (pounds and inches are converted exactly). It then looks up the CDC LMS parameters for the child’s sex and age in months — L (skewness), M (median BMI) and S (coefficient of variation) — using the CDC convention of the midpoint of the current month of age.
The LMS formula converts the BMI into a z-score, and the standard normal distribution Φ turns that into a percentile. CDC categories are: under the 5th percentile underweight, 5th to under 85th healthy weight, 85th to under 95th overweight, and 95th or above obesity. Severe obesity is 120% (class 2) or 140% (class 3) of the 95th-percentile BMI, or an absolute BMI of 35 or 40. Because the LMS curves are less reliable far above the 97th percentile, the CDC’s 2022 extended charts track very high BMIs as a percentage of the 95th percentile, which this calculator also reports.
- L
- Box-Cox power for the age and sex (skewness)
- M
- Median BMI for the age and sex
- S
- Coefficient of variation for the age and sex
- Φ
- Standard normal cumulative distribution function
Worked examples
10-year-old boy, 70 lb, 4 ft 6 in
70 lb at 54 inches gives a BMI of about 16.9. For a boy aged 10 years 0 months the CDC median BMI is about 16.6, so he sits just above the middle at roughly the 54th percentile (z ≈ 0.11) — comfortably in the healthy weight range (5th to under 85th).
14-year-old girl, 65 kg, 160 cm
65 kg at 1.60 m is a BMI of 25.4. That would be only just “overweight” for an adult, but for a girl of 14½ it lands at about the 91st percentile, which the CDC classes as overweight (85th to under 95th).
6-year-old girl, 60 lb, 3 ft 10 in
60 lb at 46 inches gives a BMI of 19.9, far above the median of about 15.2 for a 6-year-old girl. That is roughly the 97th percentile — above the 95th-percentile BMI of 18.8, so the CDC category is obesity, at about 106% of the 95th percentile (below the 120% severe-obesity line).
Frequently asked questions
Why do children use BMI percentiles instead of adult BMI categories?+
Children’s body fat changes with age and differs between boys and girls, so a single BMI cut-off would be misleading. Comparing a child’s BMI with others of the same age and sex on the CDC growth chart gives a fair screening measure.
What is a healthy BMI percentile for a child?+
The CDC defines healthy weight as a BMI-for-age from the 5th percentile up to (but not including) the 85th percentile. The 85th to under 95th is overweight, 95th and above is obesity, and below the 5th is underweight.
What does the 50th percentile mean?+
A child at the 50th percentile has a BMI higher than half of children of the same age and sex in the CDC reference population and lower than the other half — it is the median, not a target every child must hit.
What age range does this calculator cover?+
The CDC BMI-for-age charts cover ages 2 through 19 (up to 20 years). For babies and toddlers under 2 the CDC recommends the WHO weight-for-length charts instead; use our baby percentile calculator for infants.
Is BMI percentile accurate for athletic or muscular kids?+
BMI does not distinguish muscle from fat, so very muscular children and teens can show a high percentile without excess body fat. Pediatricians consider growth history, puberty stage and other measures alongside BMI.
What is severe obesity in children?+
The CDC defines class 2 severe obesity as a BMI at or above 120% of the 95th percentile for age and sex (or a BMI of 35 or more), and class 3 as 140% of the 95th percentile (or BMI 40 or more).
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.