Head Circumference-for-Age Percentile Calculator (WHO, 0-24 Months)

How Head Circumference Percentiles Work

Head circumference-for-age percentiles compare the size of your child’s head to other children of the same age and sex. The WHO growth standards use the LMS method to model the distribution of head circumference at each age, producing smooth percentile curves from birth through 24 months.

Pediatricians routinely measure head circumference because it reflects brain growth. Rapid increases or decreases in head size relative to established percentiles can signal conditions that benefit from early intervention.

Example Calculation

A 6-month-old boy with a head circumference of 43.3 cm would be approximately at the 50th percentile, since the WHO median for boys at 6 months is 43.33 cm. This means roughly half of boys the same age have a smaller head circumference and half have a larger one.

Frequently Asked Questions

How is head circumference measured?

Head circumference is measured with a flexible, non-stretchable tape placed just above the eyebrows and ears, around the widest part of the back of the head (the occiput). Your pediatrician measures this at every well-child visit during the first two years.

What does a very large or very small head mean?

A head circumference below the 3rd or above the 97th percentile may warrant further evaluation, but a single measurement is not diagnostic. Many children with large or small heads are perfectly healthy — familial factors play a significant role. The trend over time matters most.

Why does my doctor track head circumference?

Head circumference is a proxy for brain volume. Tracking it over time helps identify conditions like microcephaly (abnormally small head) or macrocephaly (abnormally large head), hydrocephalus, or craniosynostosis early, when treatment is most effective.

Are these the same standards my pediatrician uses?

Yes. The WHO growth standards used here are recommended by the CDC for all children under 2 years of age in the United States and are the global standard of care.