Few numbers reassure and confuse parents in equal measure like a growth percentile. This calculator plots your child on the same WHO and CDC charts a pediatrician uses and reads the result in plain language — but the most useful thing it can tell you is that the trend, not the single number, is what matters.
How the percentile is calculated
Modern growth charts use the LMS method. For each age and sex, three published parameters describe the distribution of a measurement: L (skew), M (the median), and S (variability). Your child's measurement is turned into a z-score with z = ((X/M)^L − 1)/(L·S), and the z-score becomes a percentile through the normal distribution.
This calculator uses the official WHO Child Growth Standards for children under 2 (weight, length, and head circumference) and the CDC growth charts for ages 2 to 20 (weight, stature, and BMI-for-age) — the same split the American Academy of Pediatrics and CDC recommend for US children.
Why WHO under 2 and CDC after
The two chart sets answer different questions. The WHO standards are prescriptive: they describe how healthy, well-nourished children should grow. The CDC charts are descriptive: they record how a reference sample of US children actually grew. Because they come from different populations, the very same measurement can land on a slightly different percentile depending on the chart — which is exactly why a small step around a child's second birthday usually reflects the switch in reference, not a real change in the child.
The trend beats the number
A single percentile is a snapshot. A baby who has steadily tracked the 15th percentile since birth is growing beautifully; a baby who slides from the 75th to the 15th over a few months is the one to ask about. Pediatricians watch for a child crossing two or more major percentile lines up or down, not for a low or high number by itself.
Plenty of perfectly healthy children sit at the 3rd or the 97th percentile — often that is simply genetics. Use this tool to understand where your child sits and to have a better-informed conversation, not to diagnose. Any real concern about growth belongs with your pediatrician, who has the full history the chart cannot show.