Infant Growth Percentile Tracker

Infant growth percentiles compare your baby’s size to other babies of the same age and sex. They are based on the WHO (World Health Organization) growth standards, built from the 2006 Multicentre Growth Reference Study, which followed breastfed babies in six countries raised in good conditions.

What percentiles mean:

• 50th percentile = Average (half of babies are larger, half are smaller)
• 25th percentile = Smaller than 75% of babies that age
• 75th percentile = Larger than 75% of babies that age
• 2nd to 98th percentile (more exactly the 2.3rd to 97.7th) = the normal band, two standard deviations either side of the median. The CDC uses these cut-offs for the WHO charts in the US.

Typical growth milestones (WHO medians):

Important notes:

• Consistent growth along ANY percentile curve is healthy
• A sudden jump or drop in percentile may warrant discussion with your pediatrician
• Breastfed and formula-fed babies may grow at slightly different rates. The WHO standard is built on breastfed babies, which is one reason formula-fed babies often sit a little higher on it after six months
• Premature babies should use adjusted age for the first 2 years, so subtract the weeks they arrived early from their calendar age before entering it

Range: this covers birth to 24 months, which is where the WHO standards use recumbent length. From 2 years the charts switch to standing height and a different reference, so the numbers here stop rather than being stretched to cover ages they were never built for. For weight alone up to 5 years, use the WHO Weight-for-Age Z-Score Calculator; from 2 to 20 years, the Child Growth Percentile Calculator uses the CDC charts.

How the percentile is worked out: baby weight is skewed, with a longer tail on the heavy side, so the WHO publishes three numbers for every month: L (the skew), M (the median) and S (the spread). The z-score is ((weight ÷ M)^L − 1) ÷ (L × S), and the percentile follows from the z-score. This page uses the official monthly L, M and S values, the same ones the CDC republishes, and draws a straight line between months for an age in between. Length has no skew (L = 1), so for length the formula is simply (length ÷ M − 1) ÷ S.

Units: 1 kg = 2.205 lbs | 1 cm = 0.394 inches