Kids BMI Calculator
Find your child's BMI-for-age percentile using the same CDC growth chart method pediatricians use, for ages 2 through 19. Unlike the fixed adult BMI scale, a child's healthy range depends on their exact age and sex.
Your result will appear here
Enter your child's age, sex, height, and weight, then click calculate to see their BMI percentile.
CalcClearly Health & Fitness Team
Content reviewed against CDC growth chart methodology
This page is researched and maintained by CalcClearly's editorial team, using the CDC's published BMI-for-age reference data and methodology. It's meant as general education, not a substitute for advice from your child's pediatrician.
Quick Answer
This calculator compares your child's BMI to the CDC's BMI-for-age growth chart reference data for their exact age (in months) and sex, and returns a percentile: the 5th-84th percentile is a healthy weight, below the 5th is underweight, the 85th-94th is overweight, and the 95th and above is classified as obesity.
How It Works: Formula & Variables
BMI = weight (kg) / height (m)²
Z = ((BMI / M)^L − 1) / (L × S)
Percentile = normal distribution CDF(Z) × 100
L, M, and S are reference values published by the CDC for each age (in months) and sex, describing the skew, median, and spread of BMI in the reference population. This calculator interpolates between the CDC's published values at 6-month age intervals.
Worked Examples
Example 1: 8-year-old boy
A boy aged 8 years 0 months, 50 inches tall (127 cm), weighing 55 lb (25 kg), has a BMI of about 15.5, which lands at roughly the 43rd percentile for his age, a healthy weight.
Example 2: 12-year-old girl
A girl aged 12 years 0 months, 60 inches tall (152 cm), weighing 119 lb (54 kg), has a BMI of about 23.4, which lands at roughly the 91st percentile for her age, in the overweight category.
Key Concepts
Percentile, not a fixed score: A child's BMI percentile changes as they grow even if their BMI number stays exactly the same, since the reference curve itself shifts with age.
Sex-specific reference data: Boys and girls have different typical growth patterns, so this calculator uses separate CDC reference tables for each.
A screening tool, not a diagnosis: Like adult BMI, a child's percentile doesn't measure body fat directly or account for build, muscle mass, or growth timing. A pediatrician considers it alongside growth history and other factors.
Common Mistakes
Applying adult BMI categories to kids: A BMI of 22 might be a healthy weight for one 10-year-old and overweight for another, depending on age and sex. There's no fixed "healthy" BMI number for children the way there is for adults.
Reacting to a single reading: Pediatricians look at growth trends over multiple visits, not one measurement in isolation. A single percentile is a data point, not a verdict.
Using this for children under 2: Infants and toddlers under 24 months are tracked on separate WHO growth charts, not the CDC BMI-for-age charts this calculator uses.
Frequently Asked Questions
Adults are compared to fixed cutoffs (18.5, 25, 30), but a child's healthy weight range changes constantly as they grow. Instead of a fixed number, a child's BMI is compared to thousands of other children of the same age and sex, using CDC growth charts, and expressed as a percentile rather than a category.
The CDC considers the 5th to 84th percentile the healthy weight range for children and teens. Below the 5th percentile is classified as underweight, the 85th to 94th percentile as overweight, and the 95th percentile and above as obesity.
It means their BMI is higher than about 75% of children of the same age and sex in the CDC reference population, and lower than about 25%. It's a relative ranking, not a pass/fail grade, and the 75th percentile still falls inside the healthy weight range (5th-84th percentile).
This calculator covers ages 2 through 19 years (24 to 240 months), matching the CDC's BMI-for-age growth charts. For children under 2, growth is tracked using WHO weight-for-length charts instead, which use a different reference population and aren't covered here.
It uses the official CDC BMI-for-age reference data (the L, M, and S parameters published by the CDC), sampled at 6-month age intervals and interpolated in between. That's a close approximation of the CDC's exact calculation, which uses slightly finer age resolution, so results may differ by a percentile point or two from a pediatrician's exact software.
A single BMI percentile reading isn't a diagnosis. Growth patterns, family history, puberty timing, and overall health all matter alongside this number. Bring the result to your child's pediatrician, who can track it over time alongside other measurements rather than react to one data point.