Head Circumference Percentile Calculator (WHO)
Enter your baby's sex, age in months (0-24), and head circumference. This uses the WHO 2006 growth standards, the reference pediatricians use for this age range.
Head circumference percentile (WHO)
56th percentile
z-score +0.14
| Age | P3 | P10 | P25 | P50 | P75 | P90 | P97 |
|---|---|---|---|---|---|---|---|
| 6 months (WHO) | 41.0 cm | 41.8 cm | 42.5 cm | 43.3 cm | 44.2 cm | 44.9 cm | 45.6 cm |
Worked example
A 9-month-old girl at 43.8cm: WHO's table gives L=1, M=43.83cm, S=0.03053 at 9 months. z = -0.02, the 49th percentile.
Frequently Asked Questions
Why does a pediatrician track head circumference separately?
It's a rough proxy for brain growth, which is fastest in the first two years. A head circumference percentile that's unusually high, unusually low, or crossing lines quickly can be an early flag for a range of conditions -- it's tracked on its own precisely because it doesn't move in lockstep with length or weight.
My baby's head is large but their length and weight are average -- is that a problem?
Not necessarily. Head size runs in families, and a head circumference a bit above the 50th percentile with average length and weight is often just normal variation, especially if it's been consistent since birth. A sudden jump across percentile lines is a different, more relevant signal than a single high reading.
Why does this stop at 24 months?
The WHO standard used here covers birth to 2 years, the period when head circumference is most clinically informative. Routine head-circumference tracking becomes far less common after age 2, so there isn't a standard CDC extension the way there is for height and weight.
How do I measure head circumference at home?
Wrap a flexible tape measure around the widest part of the head -- just above the eyebrows and ears, around the most prominent part of the back of the head -- and take the largest of three measurements. A soft cloth tape measure is easier to read accurately than a rigid one.
These calculators estimate from published population reference data (the CDC and WHO growth charts) and published rules of thumb -- not a diagnosis, and not a substitute for your pediatrician, who tracks your child's own growth curve over time and knows their history.