Newborn Weight Percentile Calculator (WHO Standards)
Calculate your baby's weight percentile based on WHO Child Growth Standards. Provides Z-Scores and growth curves for infants 0-24 months.
About
Growth monitoring is a standard pediatric practice to identify potential health issues such as failure to thrive or rapid weight gain. This calculator uses the World Health Organization (WHO) Child Growth Standards, which are the international gold standard for breastfed and formula-fed infants.
The output provides both a standard percentile (indicating what percentage of babies weigh less than yours) and a Z-Score. The Z-Score is preferred clinically as it quantifies exactly how many standard deviations a measurement is from the median (0). A Z-Score between -2 and +2 is generally considered normal.
Formulas
The calculation utilizes the LMS method, where L is the Box-Cox power, M is the median, and S is the coefficient of variation for a given age.
The Percentile (P) is derived from the standard normal cumulative distribution function (Φ):
Reference Data
| Percentile | Z-Score | Interpretation |
|---|---|---|
| 97th | +1.88 | High Weight (Monitor) |
| 85th | +1.04 | Above Average |
| 50th | 0 | Median (Average) |
| 15th | -1.04 | Below Average |
| 3rd | -1.88 | Low Weight (Monitor) |