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Growth percentile chart
Quick answer
What does a growth percentile mean?
A growth percentile is a position in a reference population, not a grade. A weight on the 30th percentile means about 30 children in 100 of the same age and sex in the reference group weigh less, and about 70 weigh more. It describes where one measurement sits on a published curve. It does not describe health.
Plot a measurement
| Age | Measurement | Percentile | z-score | Reference used |
|---|
Percentiles describe position in a reference population, not health. Growth patterns are for your pediatrician to read.
How the percentile is calculated
Both agencies publish their curves as three smoothed numbers for every age point, known as the LMS parameters: L, a power that accounts for skew, M, the median, and S, the coefficient of variation. A measurement X becomes a z-score with:
z = ((X / M)L minus 1) / (L x S)
and, in the special case where L is exactly zero, with the limit form z = ln(X / M) / S. The z-score is then read through the standard normal cumulative distribution to give a percentile: a z of 0 is the 50th percentile, a z of plus 1 is about the 84th, and a z of minus 2 is about the 2nd. The reference curves drawn on the chart are the same formula run backwards, solving for the value at a fixed z, which is how the 9 bands are produced.
The normal distribution is evaluated with the Abramowitz and Stegun rational approximation to the error function, whose worst-case absolute error is around 1.5 x 10-7, and the inverse uses Acklam's rational approximation. Both are far finer than a displayed percentile needs. Where an age falls between two published table rows, the L, M and S values are interpolated linearly between the neighbouring rows.
The 24-month reference switch
Birth through 24 completed months is plotted on the WHO Child Growth Standards, and 24 months onward on the CDC 2000 Growth Charts. That is the split US pediatric practice uses, and it is the split this tool implements literally, at exactly 24.0 months.
The two references are built from different populations, so their curves do not meet exactly at the seam. At 24 months the WHO 50th percentile weight for boys is 12.15 kg while the CDC 50th is 12.67 kg, and the WHO 50th percentile recumbent length is 87.82 cm while the CDC 50th standing stature is 86.45 cm. The length figure carries a real measurement difference too: under two, children are measured lying down, and from two they are measured standing, which makes them read slightly shorter. The chart marks the seam with a labelled line rather than smoothing it away.
Head circumference is the exception. The CDC charts carry no head-circumference reference past 36 months, so head circumference stays on the WHO standard across the whole supported range. This site never uses the CDC birth-to-36-month infant file at any age.
The reference tables this chart is drawn from
| Reference | Measurement | Age rows per sex | Granularity | Source |
|---|---|---|---|---|
| WHO | WHO Child Growth Standards (2006), weight-for-age | 25 | one row per completed month of age, 0 through 24 months | WHO published table, retrieved 2026-08-28 |
| WHO | WHO Child Growth Standards (2006), length-for-age (recumbent length, 0 to 24 months) | 25 | one row per completed month of age, 0 through 24 months | WHO published table, retrieved 2026-08-28 |
| WHO | WHO Child Growth Standards (2007 head circumference release), head-circumference-for-age | 61 | one row per completed month of age, 0 through 60 months | WHO published table, retrieved 2026-08-28 |
| CDC | CDC 2000 Growth Charts, weight-for-age (2 to 20 years) | 38 | the published CDC age grid: 24.0 months, then half-month midpoints (24.5, 25.5, ... 60.5). L, M and S at ages between grid points are linearly interpolated, which is how the CDC's own worked examples treat this file. | CDC published table, retrieved 2026-08-28 |
| CDC | CDC 2000 Growth Charts, stature-for-age (2 to 20 years) | 38 | the published CDC age grid: 24.0 months, then half-month midpoints (24.5, 25.5, ... 60.5). L, M and S at ages between grid points are linearly interpolated, which is how the CDC's own worked examples treat this file. | CDC published table, retrieved 2026-08-28 |
Every L, M and S value is reproduced from the published file exactly as the agency printed it. The full tables are readable on the reference tables page, with the exact file URL and retrieval date for each one.
Questions about this chart
What does a growth percentile actually mean?
A percentile is a position in a reference population, not a score. A weight on the 30th percentile means that if you lined up 100 children of the same age and sex from the reference group, about 30 would weigh less and about 70 would weigh more. It says where a measurement sits, and nothing about whether a child is healthy.
Is this medical advice, or a screening tool?
No. This is a visualization of two published reference datasets. It does not assess, diagnose, or screen anything, and it deliberately gives no guidance about feeding, growth patterns, or what any number means for your child. Growth is read over time and in context, which is your pediatrician's job, not a web page's.
Why does the reference change at 24 months?
Birth through 24 completed months is plotted on the WHO Child Growth Standards, and 24 months onward on the CDC 2000 Growth Charts. That split follows the recommendation US pediatric practice uses. The two are different reference populations, so the curves step slightly at the seam; the chart marks the seam rather than smoothing it away.
Does this store or upload my child's measurements?
No. The reference tables are downloaded with the page and every calculation runs in your browser. Your entries are never sent to a server and are not saved anywhere, so closing or reloading the tab clears them. Only an anonymous count that the tool was used is recorded, with no values in it.
Can I enter more than one measurement?
Yes. Add a row for each visit and the chart connects them into a trend line across the reference curves. You can enter ages in months, or enter a date of birth and the date each measurement was taken and the tool works out the age for you.
Want the statistics explained properly instead? Start with what a percentile actually means, or read why the WHO and CDC charts differ.