Head Circumference Percentile Calculator (WHO 0–5 Years)
Enter your baby’s sex, age in months and head circumference to get the WHO percentile, z-score, median and normal range for babies and children from birth to 5 years.
How it works
z = (HC ÷ M − 1) ÷ S (WHO LMS method with L = 1) · percentile = Φ(z) × 100 · M and S are interpolated between whole months
The WHO Child Growth Standards (2006) describe how healthy breastfed children grow under good conditions. For head circumference-for-age the Box-Cox power L equals 1, so the z-score is simply the distance from the median M divided by the spread M × S. A z-score between −2 and +2 (roughly the 2nd to 98th percentile) is considered normal; below −2 SD is the usual screening definition of microcephaly and above +2 SD of macrocephaly. One measurement matters less than the trend: pediatricians look at whether the head grows steadily along its own curve.
Worked example
A 6-month-old boy with a head circumference of 43.5 cm: the WHO median is 43.3 cm (M = 43.3306, S = 0.02817), so z = (43.5 ÷ 43.3306 − 1) ÷ 0.02817 = 0.14, which is percentile 55.5 — within the normal range, with a normal band of 41.0–45.6 cm.
Sources: WHO Child Growth Standards — Head circumference-for-age · CDC — WHO growth chart data files
Frequently asked questions
How do I measure my baby’s head circumference?
Use a flexible, non-stretch tape. Wrap it around the widest part of the head — just above the eyebrows and ears and around the most prominent part of the back of the head. Measure three times and use the largest value.
What is a normal head circumference percentile?
Anything between about the 3rd and 97th percentile is usually normal. What matters most is steady growth along a similar percentile line over time; crossing two major percentile lines up or down should be checked by a pediatrician.
When is a small head considered microcephaly?
Microcephaly is usually defined as a head circumference more than 2 standard deviations below the mean for age and sex (z below −2), and severe microcephaly as below −3 SD. A doctor confirms it with repeated measurements and an examination.
Should I use corrected age for a premature baby?
Yes. For babies born before 37 weeks, use the age corrected for prematurity (actual age minus the weeks born early) until about 2 years old, or use a preterm growth chart in the first weeks.
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This calculator gives general guidance and is not a diagnosis or medical advice. If you are worried about your child, or they seem unwell, contact your pediatrician, a health center or emergency services.