Head Circumference Percentile Calculator (WHO, 0–5 years)

Understanding the head circumference percentile calculator — concept and principle

This calculator compares a baby's or young child's head circumference with the WHO Child Growth Standards from birth to 5 years and gives the percentile and z-score. Parents use it to understand measurements from check-ups, and health workers use it to spot heads that are unusually small or large or are growing off track.

Boy, 18 months, 47 cm: with the default inputs the result is 39th percentile (within z −2 to +2).

How it is calculated

z = (head circumference ÷ M − 1) ÷ S

Percentile = share of the normal distribution below z

3rd and 97th percentile lines = M × (1 ∓ 1.881 × S)

M is the WHO median and S the coefficient of variation for the child's sex and age in months; values between whole months are interpolated.
For head circumference the WHO skewness parameter L is 1, so the simple formula above is exact.

The principle

The WHO standards were built from healthy, breastfed children in six countries raised in good conditions, so they describe how children should grow rather than how a particular population happens to grow. At each age the head sizes follow a nearly normal distribution with median M and spread S × M. The z-score says how many standard deviations a measurement is from the median, and the percentile translates that into the share of children who would be smaller.

Worked example

Default inputs: boy, 18 months, head circumference 47 cm.

WHO values at 18 months for boys: M = 47.37 cm, S = 0.0280.
z = (47 ÷ 47.37 − 1) ÷ 0.0280 = −0.28.
Percentile = 39; within z −2 to +2 → Yes.
Median 47.4 cm; 3rd line = 47.37 × (1 − 1.881 × 0.028) = 44.9 cm; 97th = 49.9 cm.

Each point is the calculator re-run with only “Age in months (0–60, decimals allowed)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Age in months (0–60, decimals allowed)	Head circumference percentile
4.5	100
9	94.4
13.5	66.1
18	39
22.5	21.8
27	12.1
36	4.2

How to read the result

Where does your result fall?

WHO flags z-scores below −2 or above +2 (roughly the 2nd and 98th percentiles) for a closer look. A single value outside the range may be normal, for example if parents have small or large heads. Crossing percentile lines over several visits matters more than any one percentile.

Common mistakes

Using a stretchy tape or measuring below the widest part of the head.
Using actual age instead of corrected age for babies born prematurely.
Comparing with a different chart, such as national or CDC charts, without noting it.

Keep in mind: This is not a diagnosis. Concerns about head size or growth should be discussed with your child's doctor.

Frequently asked questions

How do I measure head circumference?

Wrap a non-stretch tape around the widest part of the head, just above the eyebrows and ears, and take the largest of three readings.

Is the 10th percentile too small?

No. It is within the usual range; one in ten healthy children is smaller.

Until what age is head size checked?

Routinely in the first two years, when the head grows fastest; this chart covers up to 5 years.

Sources: WHO Multicentre Growth Reference Study Group, WHO Child Growth Standards: head circumference-for-age, arm circumference-for-age, triceps skinfold-for-age and subscapular skinfold-for-age, WHO, 2007

Checked 9 Nov 2026. WHO Multicentre Growth Reference Study Group, WHO Child Growth Standards: head circumference-for-age, arm circumference-for-age, triceps skinfold-for-age and subscapular skinfold-for-age (WHO, 2007) — head circumference-for-age LMS values, birth to 5 years; tables as distributed in the pygrowup package (hcfa_boys/girls_0_5).

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