Baby Growth Percentile Guide
This guide explains how percentiles are computed from the WHO growth standards (0-24 months) and the KCDC 2017 Korean reference — both of which use LMS parameters (Box-Cox power, Median, coefficient of variation) — and how to interpret the output. It is a reference, not a diagnosis; always confirm concerns with a pediatrician.
What is a percentile?
A percentile tells you where your baby stands relative to 100 peers of the same age and sex. E.g. the 50th percentile is the median; the 3rd percentile means 3% of peers are smaller. The 3rd-97th band (roughly -1.88 to +1.88 standard deviations) is treated as the standard reference window.
LMS method and Z-scores
WHO/KCDC charts summarize each age-sex-specific distribution with three parameters (L=skew, M=median, S=coefficient of variation). From L, M, S and the measurement x, we compute a Z-score and then a percentile from the standard normal CDF. Age is linearly interpolated across the monthly anchor rows.
z = ((x / M)^L − 1) / (L · S) (L ≠ 0) z = ln(x / M) / S (L = 0)
How to read a growth curve
The x-axis is age in months (0-24); the y-axis is the measurement. The dashed lines are the 3rd and 97th percentiles; the solid line is the 50th (median). Multiple measurements draw a trend line. If a baby consistently tracks near one curve and then crosses two major percentile bands (e.g. P75 → P25), flag it at your pediatric visit.
Interpreting the result bands
| Percentile range | Label | Meaning |
|---|---|---|
| < 3rd | Well below typical | Bottom 3% of peers. Could indicate underweight or growth concern; pediatric eval recommended. |
| 3-10th | Below typical | Lower end of the standard window. Stable trend may just be constitutional. |
| 10-25th | Low-normal | Lower half of the normal range. |
| 25-75th | Typical range | Central band; most babies sit here. |
| 75-90th | High-normal | Upper half of the normal range. |
| 90-97th | Above typical | Upper end of the standard window. Overweight is judged by weight-for-length, not weight alone. |
| > 97th | Well above typical | Top 3% of peers. Rapid climb warrants a check-in. |
Measurement tips
- Weight: fully undressed (no diaper) on an infant scale (0.01 kg). Wait at least 2 hours after feeding.
- Length (<24 months): recumbent length - measure crown to heel with legs extended flat.
- Head circumference: greatest circumference just above the eyebrows and around the occipital bulge. Average of 3 readings.
- Use the same instrument and time of day to reduce measurement error.
Corrected age (preterm babies)
For babies born 3+ weeks early, use corrected age (based on the due date) until about 24 months to avoid underestimating. This calculator uses calendar age; for preterm babies, enter the due date as the birth date to get the corrected-age result.
When to consult a pediatrician
- A single measurement below the 3rd or above the 97th percentile
- Crossing 2+ major percentile bands from baseline (e.g. P50 → P10)
- Static or declining weight (beyond a normal ±2% scale fluctuation)
- Rapid change in head circumference (rule out hydrocephalus, elevated ICP)
- Visible imbalance between weight and length (underweight / overweight)
Sources
WHO Multicentre Growth Reference Study (2006) · KCDC / Korean Pediatric Society 2017 Growth Charts · AAP Bright Futures. Last reviewed: 2026-04-19.