At almost every check-up in the first two years, a nurse or doctor wraps a tape around your baby’s head. Head circumference is one of the three basic growth measurements, with weight and length, because in infancy the head grows in step with the brain. A reading is meaningful only compared with other babies of the same age and sex, which is what the WHO growth standards provide. This calculator converts a head measurement into a percentile and a z-score and shows the range of values within two standard deviations of the median at that age.
It is an educational tool, not a diagnostic one. Head size is judged by a clinician, together with growth over time, the baby’s development and the size of the parents’ heads.
How to use the head circumference percentile calculator
- Choose your baby’s sex.
- Enter the age in months, using the corrected age for babies born early.
- Enter the head circumference in centimetres, inches or millimetres.
- Read the percentile and z-score, and the band that describes where the reading falls.
Head circumference formula
Percentile = Φ(z) × 100
Value at a given z: M × (1 + L × S × z)
L, M and S are the WHO values for each month of age and sex, interpolated between months. Φ is the standard normal distribution. The standards are in 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 (World Health Organization, 2007). WHO defines microcephaly as a head circumference more than 2 standard deviations below the median, and severe microcephaly as more than 3 below.
Worked example
A 6-month-old boy has a head circumference of 43.3306 cm.
- At 6 months the WHO boys’ median is M = 43.33 cm and S = 0.02817.
- z = (43.3306 ÷ 43.33 − 1) ÷ 0.02817 ≈ 0.00, which is the 50th percentile.
- The range within 2 standard deviations is 40.9 to 45.8 cm.
WHO head circumference by age: median and ±2 SD
Values in centimetres.
| Age (months) | Boys −2 SD | Boys median | Boys +2 SD | Girls −2 SD | Girls median | Girls +2 SD |
|---|---|---|---|---|---|---|
| 0 | 31.9 | 34.5 | 37.0 | 31.5 | 33.9 | 36.2 |
| 1 | 34.9 | 37.3 | 39.6 | 34.2 | 36.5 | 38.9 |
| 2 | 36.8 | 39.1 | 41.5 | 35.8 | 38.3 | 40.7 |
| 3 | 38.1 | 40.5 | 42.9 | 37.1 | 39.5 | 42.0 |
| 4 | 39.2 | 41.6 | 44.0 | 38.1 | 40.6 | 43.1 |
| 6 | 40.9 | 43.3 | 45.8 | 39.6 | 42.2 | 44.8 |
| 9 | 42.5 | 45.0 | 47.5 | 41.2 | 43.8 | 46.5 |
| 12 | 43.5 | 46.1 | 48.6 | 42.2 | 44.9 | 47.6 |
| 15 | 44.2 | 46.8 | 49.4 | 42.9 | 45.7 | 48.4 |
| 18 | 44.7 | 47.4 | 50.0 | 43.5 | 46.2 | 49.0 |
| 21 | 45.2 | 47.8 | 50.5 | 44.0 | 46.7 | 49.5 |
| 24 | 45.5 | 48.3 | 51.0 | 44.4 | 47.2 | 50.0 |
What the bands mean
- Within ±2 SD covers about 95 percent of healthy babies and is usually normal.
- Below −2 SD is the microcephaly range; it prompts a medical review, since there are many possible causes.
- Above +2 SD is the macrocephaly range; it is often familial but worth checking.
- Trend and context matter. A baby who has tracked a low or high percentile since birth, with normal development and parents with similar head sizes, is often perfectly healthy.
Head size should be read alongside weight and length; try the Baby Growth Percentile Calculator and Baby Weight Percentile Calculator. For ages two and over see the Child Height Percentile Calculator, and for adult-height estimates the Child Height Predictor.
This calculator is for information only. Contact your baby’s doctor, midwife or health visitor if you have any concern about your baby’s head size or development.
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. World Health Organization, 2007.
- World Health Organization. Screening, assessment and management of neonates and infants with complications associated with Zika virus exposure in utero (definition of microcephaly), 2016.
Frequently asked questions
What is a normal head circumference for a baby?
It varies with age and sex. The WHO median is about 34.5 cm for a newborn boy and 33.9 cm for a newborn girl, rising to about 43.3 cm for a boy and 42.2 cm for a girl at 6 months. Most healthy babies lie within two standard deviations of the median, roughly between the 2nd and 98th percentiles.
What is microcephaly?
The World Health Organization 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 below. It can have many causes, including infections in pregnancy and genetic conditions, and needs medical assessment. A single low reading does not mean there is a problem; measurements are repeated and interpreted with the whole picture.
What is macrocephaly?
A head circumference more than 2 standard deviations above the median is called macrocephaly. Many large heads are familial and harmless, but a clinician should assess rapid growth across percentile lines, bulging fontanelle, or other symptoms.
How is head circumference measured?
A non-stretch tape goes around the largest part of the head, just above the eyebrows and ears and across the back of the skull. Clinicians take the largest of two or three readings. Home measurements are less accurate than clinic ones.
Why does the growth trend matter more?
A baby whose head follows the 90th percentile steadily is usually fine; a head that crosses percentile lines quickly, up or down, deserves attention. Always discuss concerns with your baby’s doctor, who can plot repeated measurements.