Baby growth percentile
A rough weight and length percentile for babies from birth to two years, using WHO reference values.
What a percentile actually means
A 40th percentile weight means that out of a hundred babies of the same age and sex, about forty would weigh less and sixty would weigh more. It is a position in a distribution, not a score. A baby on the 10th percentile is not doing worse than one on the 90th; they are smaller, and smaller is a perfectly normal thing to be.
The trend is what matters
A single percentile tells you very little. What your provider watches is whether your baby is following their own curve over time. A baby who has tracked along the 25th percentile since birth is growing well. A baby who drops from the 75th to the 25th over a few months is worth looking into, even though both numbers are individually unremarkable.
How precise is this calculator?
Honestly: it is an approximation. It converts your measurement into a z-score using WHO reference means and standard deviations, then converts that to a percentile assuming a normal distribution. The official WHO standards use a method that also accounts for skewness, and infant weight-for-age is genuinely skewed, so our figure drifts from the published percentile toward the extremes. Near the middle of the range it is close. At the 3rd or 97th percentile, treat it as indicative only and use the chart from your provider.
Measuring length at home is hard
Length is notoriously difficult to measure accurately in a baby who will not straighten their legs. Small errors move the percentile a surprising amount, which is another reason to weight clinic measurements more heavily than your own.