The short answer
A height percentile and height z-score express the same age- and sex-specific reference position on different scales, and each can be converted to the other.
A percentile is usually easier to explain: the 75th percentile means the measurement equals or exceeds roughly 75% of the selected reference group. A z-score describes distance from the reference center in standard-deviation units. A z-score of 0 maps to the 50th percentile.
Z-score: standardized distance from its center.
Common z-score and percentile pairs
CDC publishes LMS values for stature-for-age and identifies these approximate pairs:
| Z-score | Percentile | Plain-language position |
|---|---|---|
| −1.881 | 3rd | Near the lower end of the reference distribution |
| −1.645 | 5th | Equals or exceeds about 5% of the reference |
| −0.674 | 25th | Below the reference median |
| 0 | 50th | At the reference median |
| 0.674 | 75th | Above the reference median |
| 1.645 | 95th | Equals or exceeds about 95% of the reference |
| 1.881 | 97th | Near the upper end of the reference distribution |
Why the conversion is not linear
Percentiles crowd together near the ends of a distribution. Moving from the 50th to the 60th percentile is not the same standardized distance as moving from the 90th to the 100th. The 100th percentile is not a finite z-score, and a percentile result should not be read as “percent of maximum height.”
CDC uses the LMS method to account for how the distribution changes by age and sex. The L, M and S parameters describe transformation, median and spread for each reference point; the calculator uses those published values to convert a standing-height measurement into a z-score and percentile.
What both numbers leave out
- They do not predict final adult height.
- They do not show how quickly height changed between measurements.
- They do not correct a poor home measurement.
- They do not diagnose a growth condition.
Use either number to describe the current reference position. Use dated, consistently taken measurements to understand the pattern, and ask a qualified clinician to interpret a concern in its full context.