Head Circumference Percentile Calculator

A head circumference percentile compares the distance around your child's head with healthy children of the same sex and age. At the 30th percentile, your child's head is larger than about 30 out of 100 of those children. This calculator uses the WHO Child Growth Standards, which give reference values for each day of age from birth to 5 years¹.

What a Head Circumference Percentile Measures

Head size is a simple outside sign of how the brain is growing, which is why it is checked at baby visits³. The percentile does not measure intelligence or development, and a small or large head on its own is not a diagnosis.

The Formula

WHO describes each day of age with three numbers: L (skew), M (median head size) and S (spread)¹. For head circumference, L is 1 at every age, so the formula becomes a plain z-score: how many standard deviations the measurement sits above or below the median.

z = ((head circumference ÷ M)^L - 1) ÷ (L × S)

With L = 1: z = (head circumference - M) ÷ (M × S)

Percentile = share of a normal bell curve below z, × 100

WHO built these standards from healthy breastfed children in six countries². For a boy aged 6 months (183 days), M = 43.3393 cm and S = 0.02817¹. At 43.0 cm (16.9 in) his z-score is -0.28, the 39th percentile.

How This Calculator Computes Your Result

Enter sex, the dates of birth and measurement (or age in years, months and days) and head circumference in cm or inches (1 in = 2.54 cm). The tool looks up that exact day in the WHO daily table, without rounding to months. You get the percentile, z-score, a short note, the median for that age and the WHO range. For babies born before 37 weeks, add weeks at birth and the tool uses corrected age until age 2.

What Your Result Means

CDC recommends the WHO charts for children under 2. It uses 2 standard deviations (the 2.3rd and 97.7th percentiles) as screening lines on these charts, not the 5th and 95th used on older CDC charts².

Median head circumference and the WHO range (-2 to +2 standard deviations) by age¹:

How to Measure Correctly

Use a tape that does not stretch. Wrap it around the widest part of the head: across the forehead just above the eyebrows, above the ears, and over the most rounded part of the back of the head⁴. Read it to the nearest 0.1 cm, measure three times and use the largest number⁴.

At birth, the head can be shaped by delivery. Birth charts are based on measurements taken in the first 24 hours, and a doctor may wait longer if delivery changed the head shape⁵. Small errors matter: at 6 months, one standard deviation is only about 1.2 cm¹, so a half-centimeter slip can move the percentile by about 15 points.

Microcephaly and Macrocephaly: What the Cutoffs Mean

WHO defines microcephaly as a head circumference more than 2 standard deviations below the median for age and sex, and severe microcephaly as more than 3 below³. Macrocephaly is a head circumference more than 2 standard deviations above the mean⁶. These are the lines in the first table above.

About 2 in 100 healthy children fall beyond each line by definition. Many children with a large head are healthy, and a large head often runs in families, so doctors may measure the parents' heads too⁶. A low or high reading is a reason for a careful repeat measurement and a talk with your child's pediatrician, not a diagnosis.

Why the Trend Matters More Than One Measurement

Doctors look at how head size grows across visits, and WHO advises measuring it at each visit to follow brain growth³. A review of macrocephaly notes that serial measurements are needed through 24 to 36 months, and that crossing one or more major percentile lines calls for a closer look⁶.

Head growth is fastest early on: about 2 cm a month from birth to 3 months, about 1 cm a month from 3 to 6 months, and about 0.5 cm a month in the second half of the first year⁶. It then slows to about 1 cm every 6 months until age 3⁶.

Babies Born Early

The WHO sample left out babies born before 37 weeks². Premature babies are plotted by corrected age (actual age minus the weeks born early), usually until age 2, including head circumference⁷. A girl born at 30 weeks who is 120 days old with a 37.0 cm head is on the 28th percentile at her corrected age of 50 days; by actual age she would show below the 1st. The WHO standard starts at term, so the tool gives no result before the due date.