Child BMI Percentile Calculator (CDC)
Find a child’s BMI-for-age percentile from the CDC growth charts for ages 2 to 19. Enter age, sex, height and weight to get the percentile, z-score and CDC weight category, with the CDC extended method above the 95th percentile.
How to Use
- Choose how to give the age: years and months, or the birth date and the date of the measurement (left blank, the measurement date is today).
- Pick boy or girl. Girls and boys have separate CDC charts because they grow differently.
- Enter the height (cm, m, inches or feet and inches) and the weight (kg, lb or stone), measured on the same day.
- Read the BMI-for-age percentile, the CDC weight category, the BMI and the z-score. At or above the 95th percentile you also get the percent of the 95th percentile and the obesity class.
- The growth chart shows the 5th, 50th, 85th and 95th percentile curves from 2 to 20 years with the child marked; Show Work lists the CDC L, M and S values used.
Worked Example
A 10-year-old boy. 140 cm and 33 kg gives a BMI of 33 ÷ 1.4² = 16.84. At exactly 120 months the CDC values, halfway between the 119.5- and 120.5-month rows, are L = −2.77369, M = 16.6246 and S = 0.119794. Then z = ((16.84 ÷ 16.6246)^−2.77369 − 1) ÷ (−2.77369 × 0.119794) = 0.10, and Φ(0.10) puts him at percentile 54.1: a healthy weight. For his age the 85th-percentile BMI is 19.36 and the 95th is 22.11.
Above the 95th percentile. A boy of 14 years 3 months, 165 cm and 80 kg, has a BMI of 29.38, above the 95th-percentile BMI of 26.22 for his age. The CDC extended method uses σ = 0.3728 + 0.5196 × 14.25 − 0.0091 × 14.25² = 5.929, so the percentile is 90 + 10 × Φ((29.38 − 26.22) ÷ 5.929) = 97.0. He is at 100 × 29.38 ÷ 26.22 = 112.1% of the 95th percentile: obesity, class 1.
The common mistake: reading a child’s BMI with the adult table. A girl of 6 years 6 months, 118 cm and 21 kg, has a BMI of 15.08. The adult table would call that underweight (under 18.5), but for her age and sex it is percentile 43.9, comfortably in the healthy range. In the other direction, a 12-year-old boy with a BMI of 33.33 would be adult obesity class I, but at 137.8% of his age’s 95th percentile he is in the CDC’s severe obesity group.
Show Work
Formulas
The CDC Growth Charts and the LMS Method
The CDC growth charts used in the United States were published in 2000, built by the National Center for Health Statistics from five national health examination surveys run between 1963 and 1994 (Kuczmarski and colleagues, 2002). They include BMI-for-age charts for boys and girls from 2 to 20 years, the first US charts to do so.
The curves are smoothed with the LMS method of Tim Cole and Peter Green (1992): at each age a Box-Cox power L straightens out the skew of the BMI distribution, M is the median and S the coefficient of variation, so any BMI turns into a z-score and a percentile with one formula. The table this calculator uses is the CDC’s own reference file from its cdcanthro software.
Because few children in the old surveys had very high BMIs, the LMS curves are unreliable far above the 95th percentile. In 2020 Rong Wei and CDC colleagues published the extended method, which measures the distance above the 95th percentile with an age-dependent spread, and the CDC released extended BMI-for-age charts in 2022. The severe-obesity classes based on percent of the 95th percentile are those the American Academy of Pediatrics uses and the CDC lists.
About This Tool
This calculator works out a child’s BMI from height and weight in metric, US or stone units, takes the age from years and months or from two dates, and reads the CDC 2000 BMI-for-age table for that age and sex, interpolating between months exactly as the CDC’s cdcanthro code does. It gives the percentile, z-score and CDC weight category, switches to the CDC extended percentile at the 95th and above, and draws the child on the 5th, 50th, 85th and 95th percentile curves. Show Work lists every table value used, so the result can be checked by hand. It covers ages 2 to 19; from 20, use the adult BMI Calculator.
A percentile from one visit is a screening measure. Children grow in spurts, and doctors look at how a child tracks along the curves over time, at their height, puberty and family, before drawing conclusions. For general information only — not medical advice. Talk to a doctor or other qualified professional about your own health or your child’s.
Everything runs in your browser; nothing you enter is sent anywhere.
Related tools: BMI Calculator, Height Calculator, and Body Fat Calculator.
Frequently Asked Questions
How is a child’s BMI percentile worked out?
The CDC charts give three numbers for every age, L, M and S. The z-score is ((BMI ÷ M)^L − 1) ÷ (L × S), and the percentile is the share of the normal curve below it. A 10-year-old boy with a BMI of 16.84 has M = 16.62 at 120 months, a z-score of 0.10 and a percentile of 54.1.
What are the CDC weight categories for children?
Underweight is below the 5th percentile, healthy weight is the 5th to below the 85th, overweight is the 85th to below the 95th, and obesity is the 95th percentile or above. Severe obesity is 120% of the 95th-percentile BMI or more, or a BMI of 35 or more; for a 12-year-old boy the 95th percentile is a BMI of 24.19, so severe obesity starts at 29.03.
Why does the percentile change method above the 95th percentile?
The 2000 charts were built from data with few very high BMIs, so above the 97th percentile the usual LMS curve squeezes very different children into nearly the same number. The CDC extended method measures the distance above the 95th percentile instead. A 12-year-old boy with a BMI of 33.33 is at percentile 99.27 by LMS and 99.58 by the extended method, at 137.8% of the 95th percentile.
Why can’t I use the adult BMI categories for a child?
Children’s BMI changes with age: it falls in early childhood and rises through the teens. A 6½-year-old girl, 118 cm and 21 kg, has a BMI of 15.08, which the adult table would call underweight, but for her age and sex it is percentile 43.9: a healthy weight.
How exact does the age need to be?
Months matter. A girl who is 145 cm and 45 kg (BMI 21.40) is at percentile 91.4 at exactly 10 years but 89.7 at 10 years 6 months, because the curves rise with age. With dates, age in months is the days between them ÷ 30.4375, the average month.
How do I use the Child BMI Percentile Calculator (CDC)?
Simply type your numbers and read the result, which refreshes the instant you change something. There is nothing to submit and nothing to wait for.
Is it free? Does it work without internet?
Yes to both. It is free with no sign-up, and once the page has loaded it keeps working even with no internet.
Where does my data go?
Nowhere — every calculation runs on your own device. Nothing you enter is uploaded, logged, or stored.
Common Use Cases
After a check-up
A school nurse records 145 cm and 45 kg for a girl born 15 March 2016, measured 4 October 2026: 126.65 months, BMI 21.40, percentile 89.5, overweight.
Underweight teens
A 16-year-old girl at 160 cm and 42 kg has a BMI of 16.41, percentile 3.1, below the 5th-percentile BMI of 16.77 for her age: CDC underweight.
US units
An 8-year-old boy, 4 ft 2 in and 60 lb, has a BMI of 16.87, percentile 72.5.
Tracking obesity treatment
Above the 95th percentile, the percent of the 95th is easier to follow over time: a 14-year-3-month boy at 165 cm and 80 kg is at 112.1%, obesity class 1.
Parents and teens
A 15-year-old boy at 165 cm and 60 kg has an adult-looking BMI of 22.04; for his age it is percentile 75.8, a healthy weight.
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