Head Circumference Percentile Calculator (0–24 months)
Enter your child's age, sex and measured head circumference to calculate their percentile on the WHO Child Growth Standards chart — instantly seeing where they fall on the bell curve.
Sex
months
cm
WHO reference, boys, age 6 months
- 1
Standard deviation
M × S = 43.35 × 0.0278 = 1.2051SD = M × S from the interpolated WHO LMS parameters at this age. - 2
Z-score
(43.5 − 43.35) ÷ 1.2051 = 0.124 - 3
Percentile
Φ(0.124) × 100 = 57Φ is the standard-normal CDF; values between −1.88 and +1.88 correspond to the 3rd–97th percentile range.
How does this calculator work?
Calculate a baby's head circumference percentile using WHO Child Growth Standards (0–24 months): Z = (HC − median) / SD, then convert to percentile using the normal distribution. A result between the 3rd and 97th percentile (Z between −1.88 and +1.88) is within the typical range. Trends across measurements matter more than any single number.
Formula
How this is calculated
The World Health Organization Child Growth Standards (2006) define reference distributions for head circumference (occipitofrontal circumference) from birth to 24 months, separately for boys and girls. The distributions are described by three parameters per age: L (Box-Cox power), M (median), and S (coefficient of variation). For head circumference the L parameter is close to 1 (the distribution is approximately normal), so this calculator uses a simplified z-score: Z = (measured HC − M) / (M × S), where M and S are interpolated from the WHO table at the child's exact age in months. The standard-normal CDF then converts Z to a percentile.
Because the L value is approximated as 1, the formula is most accurate near the median and slightly under-estimates percentiles in the far tails (below the 3rd or above the 97th percentile). This is sufficient for the health-monitoring purpose of the tool. Head circumference measurements should be taken with a non-stretch tape measure placed around the widest part of the head — just above the eyebrows, above the ears, and around the most prominent part of the back of the skull.
A single percentile is less informative than the trend over several measurements. Paediatricians are primarily concerned when a child's percentile changes markedly across visits, or when the value falls below the 3rd or above the 97th percentile, prompting further evaluation.
Frequently asked questions
Any percentile between the 3rd and 97th (roughly ±2 standard deviations from the median) is considered within the normal range. A single measurement outside this range warrants evaluation but is not automatically cause for concern — trends across multiple visits matter more.
A head below the 3rd percentile is called microcephaly; above the 97th is macrocephaly. Both can be normal variants (e.g. familial head size) or indicate underlying conditions. A paediatrician evaluates head shape, neurological development, family history and growth trends before drawing conclusions.
The WHO Multicentre Growth Reference Study followed children from birth to 5 years, but head circumference growth slows considerably after age 2 and the WHO standards for this measurement only extend to 24 months. For older children the CDC 2000 growth charts extend to 20 years.
Also known as
TG we-Calculate Editorial Team. (2026). Head Circumference Percentile Calculator (0–24 months) [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/head-percentile-calculator
TG we-Calculate Editorial Team. "Head Circumference Percentile Calculator (0–24 months)." TG we-Calculate. 2026. https://we-calculate.com/calculator/head-percentile-calculator.
TG we-Calculate Editorial Team, "Head Circumference Percentile Calculator (0–24 months)," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/head-percentile-calculator
@misc{wecalculate_head_percentile_calculator, title = {Head Circumference Percentile Calculator (0–24 months)}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/head-percentile-calculator}}, year = {2026}, note = {TG we-Calculate} }
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