Baby Percentile Calculator — WHO 2006 Growth Standards

Formula

How this is calculated

This calculator uses the WHO Child Growth Standards (2006), the international reference published by the World Health Organization from a study of healthy children raised in optimal conditions across six countries. The LMS method (Box-Cox power L, median M, coefficient of variation S) corrects for the slightly right-skewed shape of weight distributions: Z = [(X/M)^L − 1] / (L × S) transforms the measurement to a standard normal Z-score, and the normal cumulative distribution function converts that Z-score to a percentile.

The LMS parameters are interpolated linearly between reference points at 0, 3, 6, 9, 12, 18 and 24 months for both sexes. The interpolated values closely match the official WHO month-by-month tables but may differ by a small amount at intermediate ages. The bell-curve shown is a normal approximation (weight distributions are slightly right-skewed at the extremes); the red marker shows where your baby falls, and the shaded area represents the fraction of the reference population who weigh less — that fraction is the percentile.

A single percentile reading is a snapshot, not a diagnosis. Healthy babies may sit consistently at the 5th or 95th percentile for years. What matters clinically is the trend: a drop of more than 20 percentile points across two consecutive measurements, or a weight persistently outside the 3rd–97th range with other symptoms, warrants a paediatrician review. WHO 2006 standards apply to 0–5 years; the CDC 2000 charts are used in the US from 24 months onward.