Head Circumference Percentile Calculator

What Is Head Circumference?

Head circumference (HC), also called occipitofrontal circumference (OFC), is a measurement of the widest part of the head.
It is measured with a flexible tape over the forehead (supraorbital ridges) and the occipital prominence at the back of the skull.
Head circumference is a standard newborn and infant measurement, typically taken at every well-child visit up to age 2–3 years.
It reflects brain growth, because the skull expands to accommodate the growing brain.

Why It Matters

An HC more than 2 standard deviations below the mean (about the 2nd percentile) is called microcephaly; some clinics use the 3rd percentile instead.
Microcephaly can indicate genetic conditions, congenital infections (Zika, CMV, toxoplasmosis), or fetal alcohol syndrome.
An HC more than 2 standard deviations above the mean is macrocephaly.
Isolated macrocephaly is often familial (benign familial macrocephaly) but can indicate hydrocephalus or storage diseases.
Tracking the trend over time is more important than a single measurement.

WHO Growth Standards

The World Health Organization (WHO) 2006 Multicentre Growth Reference Study established growth standards for children 0–5 years.
These standards were developed from children in Brazil, Ghana, India, Norway, Oman, and the United States, all raised in healthy conditions.
This calculator uses the WHO’s own monthly table, which gives a median (M) and a spread (S) for every month from birth to 60 months. Head size is not skewed, so the z-score is simply (HC ÷ M − 1) ÷ S, with a straight line between months for an age in between.
In the US, the CDC recommends the WHO charts up to age 2; its older CDC 2000 head circumference chart runs only to 36 months.

Normal Development (WHO medians)

At birth: 34.5 cm for boys, 33.9 cm for girls.
At 1 month: 37.3 / 36.5 cm. At 6 months: 43.3 / 42.2 cm. At 1 year: 46.1 / 44.9 cm. At 2 years: 48.3 / 47.2 cm.
The head grows fastest in the first year, going from 34.5 cm to about 46 cm, close to 12 cm in 12 months.
After age 2, head growth slows sharply. A boy’s median adds only about 2.5 cm between 2 and 5 years.

Interpreting Percentiles

50th percentile: the median. Half of healthy children measure above it and half below.
2nd to 98th percentile (z −2 to +2): the normal range on the WHO charts.
Outside it: warrants evaluation, not automatic diagnosis.
Crossing 2 or more major percentile lines (e.g., 75th → 25th) in a short time is clinically significant.

Important Limitations

This calculator provides reference percentiles for screening purposes only.
Always use calibrated measuring equipment and proper technique. A tape that slips half a centimetre moves a baby’s percentile by 10 points or more.
A pediatrician should evaluate any measurement outside the normal range or any concerning trend.
Ethnic and familial variation in head size is normal, so measuring both parents is often the first step with a large head.