Calculators
Head circumference calculator
The measurement taken at every well-child visit, placed against the reference. Official standards from 7 countries and regions, the furthest reaching 18 years.
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How to read this number
Head circumference measures the skull, and the skull grows with the brain. That is why it is taken at every visit before age three, and why many standards stop publishing it after that: growth falls below a centimetre a year, less than the error in a single measurement. Coverage therefore varies a lot between standards, and this page states where the one you picked stops rather than extrapolating past it.
A single head-circumference figure says very little on its own. Two things carry the meaning: how it changes over time, and whether it fits the child’s height and weight. A small head on a small child and a small head on an average-sized child are not the same finding — and one percentile cannot tell them apart.
How you measure decides whether the number means anything: run the tape above the brow ridges and around the most prominent point at the back of the head, taking the largest circumference, snug but not tight. Flatten thick hair. Half a centimetre is several percentiles in infancy.
This page gives a position, not a verdict. A head circumference at either extreme has many possible explanations and has to be read with the birth history, the other measurements and how the child is developing — which is what a paediatrician does in a consultation, and not something a web page replaces.
The standard behind it
This calculator uses WHO Child Growth Standards (2006) · WHO Growth Reference (2007)’s LMS parameters. Ages 0–5 use the 2006 Child Growth Standards and 5–19 the 2007 growth reference; the two join at five.
The source tables are taken straight from who.int, data version 2026-09-03. Ages beyond what a standard covers are never extrapolated — the page says there is nothing there.
Common questions
- At what age do you stop measuring head circumference?
- The standards disagree: China’s WS/T 423 stops at 3 years, WHO at 5, while the Hong Kong and UK references run to 18. This page reports where the standard you picked ends and says so plainly rather than extrapolating.
- Is a small or large head circumference something to worry about?
- This page cannot answer that. A percentile is a position; what matters for head circumference is the trend and whether it fits height and weight. Taking several dated measurements to a paediatrician is far more use than a verdict drawn from one figure.
- Why does my measurement differ from the clinic’s?
- Usually the tape sat differently. The standard method runs above the brow ridges and around the most prominent point at the back, taking the largest circumference. A slightly different path or a looser tape easily shifts it half a centimetre — several percentiles in infancy.
- How does this work for a preterm baby?
- The same way as height and weight: look the child up by corrected age, not by age since birth. There is a separate corrected-age calculator on this site.