Growth Percentiles: From Measurement to Z-Score
A percentile sounds mysterious until you see the z-score behind it. This post explains the median, the spread, and the curve that links them.
What is the Growth Percentile Calculator?
Clinical growth charts give a median and a spread for each age and sex. Enter a measurement together with the median and standard deviation from the chart you are using, and the tool computes a z-score and an approximate percentile. This calculator does not contain chart data itself, so it cannot diagnose growth issues — check with a clinician for any concern.
How to use it
- Enter the child's measurement.
- Enter the chart median in the same unit.
- Enter the chart standard deviation and click Calculate Percentile.
Worked example
A measurement of 100 against a median of 95 with a spread of 4 gives a z-score of 1.25 and an approximate percentile of 89.44.
Does a low percentile mean a problem?
Not on its own. Children grow on many curves, and a single point matters less than the trend over time and a clinician's view.
Frequently asked questions
Where do I get the median and spread?
From the growth chart for the child's age and sex, or from your clinician, who can read the chart with you.
Can I use this for weight too?
Yes, as long as the measurement, median, and spread all use the same unit.