About GrowthPercentile.com

A free, open tool for parents and clinicians. Here's where the data comes from, how percentiles are calculated, and what the numbers mean.

Data Sources

All calculations use official reference data published by two organizations. No proprietary datasets or modifications are applied — the LMS parameters are used exactly as published.

WHO Child Growth Standards (Birth – 5 Years)

The WHO Child Growth Standards are based on the Multicentre Growth Reference Study (MGRS), which followed approximately 8,500 healthy breastfed children from six countries (Brazil, Ghana, India, Norway, Oman, and the United States) between 1997 and 2003. These standards represent how children should grow under optimal conditions regardless of ethnicity or socioeconomic status.

Charts available from WHO data on this site:

WHO data was downloaded in tabulated format (monthly intervals) from the official WHO website and converted to JSON using a Python script. The original LMS parameter values are preserved without rounding or modification.

CDC Growth Charts (Birth – 20 Years)

The CDC 2000 Growth Charts are based on data from five US National Health Examination Surveys (NHES) and National Health and Nutrition Examination Surveys (NHANES) collected between 1963 and 1994. These charts describe how American children actually grew — a descriptive reference, not a prescriptive standard.

Charts available from CDC data on this site:

CDC LMS data was downloaded as CSV files from the CDC website and converted to JSON. Duplicate header rows present in some CDC files were automatically removed during conversion. The original L, M, and S parameter values are preserved exactly.

Which Chart Is Used When?

Following the recommendation of the American Academy of Pediatrics (AAP):

When both data sources are available for a child's age, both results are displayed. The recommended chart for that age range is clearly marked.

Calculation Methodology: The LMS Method

Both WHO and CDC growth charts use the LMS method developed by Tim Cole (1990). This statistical technique summarizes the distribution of a measurement (weight, height, BMI) at each age using three parameters:

These three values are pre-computed and published for each age/sex combination in the reference tables. Given a child's measurement, we compute a z-score using the following formula:

When L ≠ 0:

Z = ((X / M)L − 1) / (L × S)

When L = 0:

Z = ln(X / M) / S

Where X is the child's measurement (weight in kg, height in cm, or BMI in kg/m²).

Interpolation

When a child's exact age falls between two reference data points, the calculator uses linear interpolation on the L, M, and S parameters individually, then computes the z-score from the interpolated values. This provides smooth results at any fractional age rather than snapping to the nearest reference point.

Z-Score to Percentile Conversion

The z-score is converted to a percentile using the cumulative distribution function (CDF) of the standard normal distribution. This is implemented with an approximation based on the Abramowitz and Stegun method (Handbook of Mathematical Functions, 1964). A z-score of 0 corresponds to the 50th percentile (median), +1 to approximately the 84th percentile, and −2 to approximately the 2.3rd percentile.

BMI Calculation

Body Mass Index is calculated as:

BMI = weight (kg) / height (m)²

The resulting BMI value is then compared against CDC age-and-sex-specific LMS parameters to produce a percentile. BMI percentiles are only meaningful for children aged 2 to 20 years.

Height Prediction

The height predictor uses two methods:

Interpreting Percentile Results

A percentile indicates the proportion of the reference population that a child's measurement equals or exceeds. For example, a child at the 75th percentile for weight weighs more than 75% of children of the same age and sex in the reference population.

Percentile Range Z-Score Range Interpretation
< 3rd < −1.88 Below normal range — discuss with pediatrician
3rd – 15th −1.88 to −1.04 Low-normal — may warrant monitoring
15th – 85th −1.04 to +1.04 Normal range
85th – 97th +1.04 to +1.88 High-normal — may warrant monitoring
> 97th > +1.88 Above normal range — discuss with pediatrician

Single readings vs. trends: A single percentile reading is far less informative than a series of readings over time. A child consistently tracking at the 10th percentile is growing normally — they are simply smaller than average. Pediatricians watch for percentile crossing (moving across two or more major percentile lines over several visits), which can indicate a growth concern worth investigating.

BMI Percentile Categories (Ages 2–20)

The CDC defines the following BMI-for-age categories for children and teens:

Accuracy & Limitations

Privacy & Data Handling

GrowthPercentile.com performs all calculations entirely in your browser. No measurement data is transmitted to any server. No cookies are set for tracking purposes. The only data stored locally is an optional visit counter using your browser's localStorage, which you can clear at any time through your browser settings.

References

Educational tool — not medical advice. Verify with a qualified healthcare professional before making clinical decisions.

About This Calculator — FAQ

Is this calculator a replacement for my pediatrician?
No. This is an educational tool that uses the same reference data your pediatrician uses, but it cannot replace clinical judgment. Growth assessment involves more than a single percentile number — your pediatrician considers your child's medical history, family genetics, feeding patterns, development milestones, and the trend across multiple visits.
Where does the data come from?
All data comes directly from the World Health Organization (WHO) and the US Centers for Disease Control and Prevention (CDC). The LMS parameter tables are used exactly as published — no values have been modified.
How accurate are the results?
The calculations match the results you would get from the official WHO and CDC percentile calculators. The LMS method and z-score to percentile conversion are implemented to high precision. The main source of variation is measurement accuracy — small differences in how weight or height is measured will affect the percentile result.
Is my child's data stored or shared?
No. All calculations happen entirely in your browser. No measurement data is sent to any server. Nothing is stored except an optional visit counter in your browser's local storage, which you can clear at any time.
Why do WHO and CDC give different percentiles for the same child?
The two datasets use different reference populations. WHO standards are based on healthy breastfed children from six countries and represent optimal growth. CDC charts are based on US national survey data and describe how American children actually grew. Differences of 5–15 percentile points between the two are common and expected, especially in the first two years of life.
What is the LMS method?
The LMS method is a statistical technique developed by Tim Cole in 1990 that summarizes a growth distribution using three parameters: L (Box-Cox power for normalization), M (median), and S (coefficient of variation). These parameters allow any measurement to be converted to a z-score and then to a percentile, accounting for the fact that growth data is often skewed (not perfectly symmetrical).