BMI Percentile Calculator for Kids & Teens

Find a child's BMI-for-age percentile (ages 2 to 20) from CDC growth charts, including CDC's extended method for BMIs above the 95th percentile.

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Why children get a percentile, not just a BMI

An adult BMI of 25 means the same thing at 30 or 60. A child's BMI does not work that way. It normally falls from about age 2 to age 5 or 6, then climbs through the teen years, and the curve differs for boys and girls. A BMI of 18 is high for a 4-year-old boy and ordinary for a 14-year-old girl.

That is why the CDC compares a child's BMI with children of the same sex and age in months. The result is a percentile: a boy at the 70th percentile has a higher BMI than 70 out of 100 boys his age in the CDC reference population. The categories come straight from those percentiles:

  • Underweight: below the 5th percentile
  • Healthy weight: 5th to less than the 85th
  • Overweight: 85th to less than the 95th
  • Obesity: 95th percentile or higher
  • Severe obesity: BMI at or above 120% of the 95th percentile

How the calculator works out the percentile

Step 1, BMI. Weight in kilograms divided by height in meters squared. A 10-year-old who weighs 70 lb (31.8 kg) and is 4 ft 7 in (1.397 m) has a BMI of 16.3. Imperial inputs are converted first, so pounds and centimeters give the same answer.

Step 2, up to the 95th percentile. The calculator looks up the CDC 2000 growth chart parameters L, M and S for the child's sex and month of age and computes a z-score as ((BMI / M)^L - 1) / (L x S). The z-score is converted to a percentile with the normal distribution. M is the median BMI for that age.

Step 3, above the 95th percentile. The original 2000 charts were never meant to describe very high BMIs, and their top percentiles bunch together. CDC's Extended BMI-for-age charts, published in 2022, replace that tail with a different formula: percentile = 90 + 10 x the normal CDF of (BMI - P95) / sigma, where sigma is an age- and sex-specific spread parameter. This keeps percentiles and z-scores meaningful for children with severe obesity. The calculator also shows BMI as a percent of the 95th percentile, which is how clinicians describe severity.

All parameters are taken from CDC's public data file for the extended charts, which also includes the 2000 LMS values below the 95th percentile.

Common mistakes when checking a child's BMI percentile

  • Using the adult BMI categories. A BMI of 24 is listed as healthy for adults but can be well above the 95th percentile for a 9-year-old. Always use the percentile for anyone under 20.
  • Rounding the age to whole years. Children's BMI changes month to month, so the calculator uses years plus months. A child who is 7 years 11 months is compared with a different chart row than one who just turned 7.
  • Measuring height with shoes on. Half an inch of heel lowers the BMI and can move a child across a category line. Measure standing against a wall, heels together, without shoes.
  • Reading one number as a diagnosis. BMI does not separate muscle from fat, and growth spurts can shift a percentile for a while. Pediatricians look at the trend over several visits.
  • Using it for children under 2. Infants and toddlers are assessed with weight-for-length charts instead.

Frequently Asked Questions

Common questions about the BMI Percentile Calculator for Kids & Teens

CDC classifies the 5th percentile up to, but not including, the 85th percentile as a healthy weight for children and teens ages 2 to 19.

Sources & References

Data file for the Extended CDC BMI-for-age Growth Charts for Children and Adolescents

L, M, S and sigma parameters by sex and month of age, and the formulas for percentiles below and above the 95th percentile used by this calculator.
→Source: Centers for Disease Control and Prevention

CDC Growth Charts: Percentile Data Files with LMS Values

CDC 2000 BMI-for-age LMS parameters and the LMS z-score method.
→Source: Centers for Disease Control and Prevention

BMI Categories for Children and Teens

Percentile cut-offs for underweight, healthy weight, overweight, obesity and severe obesity.
→Source: Centers for Disease Control and Prevention

Wei R, Ogden CL, Parsons VL, Freedman DS, Hales CM. A method for calculating BMI z-scores and percentiles above the 95th percentile of the CDC growth charts. Ann Hum Biol. 2020;47(6):514-521

Derivation of the extended percentile method (90 + 10 x normal CDF of (BMI - P95) / sigma).
→Source: Annals of Human Biology (PubMed)

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