About ChartMyBaby

ChartMyBaby is a free, independent tool that helps parents understand where their child sits on the standard growth charts — the same weight-for-age and height-for-age percentiles a pediatrician uses. No sign-up, no cost.

What data it uses

All percentiles come from the WHO Child Growth Standards (2006) — the international reference published by the World Health Organization, based on data from healthy, well-nourished children of diverse backgrounds around the world. We use the official published values; ChartMyBaby is not affiliated with or endorsed by the WHO.

The calculator uses weight-for-age (birth to 5 years), length-for-age for children under 2 (measured lying down), and height-for-age for children 2 and older (measured standing) — switching automatically at 24 months, exactly as the WHO standards define.

How percentiles are calculated

The WHO standards are published as LMS parameters — three numbers for each age and sex: L (a skew factor), M (the median), and S (the coefficient of variation). For a given measurement, the calculator computes a z-score with the standard LMS formula:

z = ((measurement / M)^L − 1) / (L × S)

That z-score is then converted into a percentile using the normal distribution. So a result of "50th percentile" means the child is at the median; "75th" means they're larger than about 75% of children the same age and sex. This is the same method behind official WHO and pediatric growth charts.

What it is (and isn't)

ChartMyBaby is an educational tool for tracking growth between check-ups. It is not medical advice and doesn't replace your pediatrician or health visitor. Percentiles describe position, not health — healthy children come in all sizes, and the trend over time matters more than any single number. Please read our full disclaimer.