Child BMI Percentile Calculator

BMI-for-age percentile for children and teens from 2 to 19, using CDC growth chart data.

What it does

Body mass index for a child is a single number that means little on its own. A BMI of 18 is high for a 5 year old and ordinary for a 15 year old. The useful comparison is with other children of the same age and sex, which is what a BMI-for-age percentile does.

The CDC growth charts provide reference data for ages 2 through 19. For each half month of age and each sex, the chart stores three values known as L, M, and S. M is the median BMI, S controls the spread, and L shapes the skew. Together they turn a raw BMI into a z score, and the z score turns into a percentile.

The percentile answers a direct question: what share of children of this age and sex have a lower BMI? A boy at the 70th percentile has a higher BMI than about 70 out of every 100 boys his age. The category follows the percentile: under the 5th is underweight, 5th to under 85th is healthy weight, 85th to under 95th is overweight, and 95th or above is obesity.

The metric and imperial modes do the same math. Height and weight are converted to centimeters and kilograms before BMI is computed, so a child measured in inches and pounds lands on the same percentile as one measured in centimeters and kilograms. The default is metric because pediatric growth charts and most clinical records outside the United States use it.

The CDC percentile is a screening tool, not a diagnosis. It does not measure body fat directly, and a muscular child can land above the healthy band. A clinician looks at growth over time, family history, and other measures before drawing conclusions.

How to use

1. Choose metric or imperial units.
2. Select the child's sex, since the reference curves differ.
3. Enter the age in whole years and additional whole months.
4. Enter height and weight in the chosen units.
5. Read the BMI, percentile, z score, and category.
6. Compare the result with earlier measurements to see the growth trend rather than a single point.

Formula

BMI

Weight in kilograms divided by height in meters squared.

L

Box-Cox power from the CDC LMS table for the child's sex and age in months.

M

Median BMI for that sex and age, from the same table.

S

Coefficient of variation for that sex and age.

z

Standard deviation score, later converted to a percentile with the normal distribution.

The calculator looks up the CDC row for the age in months plus 0.5, matching the half-month grid of the reference file. Ages below 24 months use the infant charts, which are based on length and a different method, so this page starts at 2 years. At 20 years the adult BMI categories take over. The CDC extended BMI method for values far above the 95th percentile is not applied here; very high BMIs are reported with the standard LMS formula.

Worked example

Reference table

Limits

• Applies only from age 2 through 19 years; infants and adults use different references.
• Screens for weight status and does not measure body fat or diagnose a health condition.
• Uses the CDC 2000 reference population, which may not match every country or population.
• Does not apply the CDC extended BMI method for severe obesity above the 95th percentile.

Common mistakes

• Reading a child's BMI against adult cutoffs such as 18.5 and 25.
• Entering feet and inches as a single decimal instead of total inches.
• Forgetting that the reference curves differ by sex.
• Comparing a single measurement with a chart instead of tracking the trend over years.
• Using the infant chart for a child who is already 2 years old.

Try it yourself

Quick checks. Answers sit tucked under each question.

Questions

Sources

• CDC: growth chart data files, including BMI-for-age LMS tables
• CDC: extended BMI-for-age growth charts
• CDC: measuring children and using growth charts

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