Head circumference percentile calculator
See where your baby's head measurement sits on the WHO head-circumference-for-age curves, and what's typical at each month.
WHO is what US paediatricians use under 2. For premature babies, percentiles use corrected age until 2 years.
Enter head circumference to see the percentile and where it sits on the WHO curves.
What a percentile actually means
Imagine 100 healthy, breastfed six-month-old girls from the WHO study, lined up from lightest to heaviest. A baby "on the 30th percentile" is heavier than about 30 of them and lighter than about 70. Drag to pick a baby.
- The 50th isn't the "goal". The 10th and the 90th percentile are both perfectly healthy positions. Half of all healthy babies are below the median.
- Percentiles squash at the edges. Moving from the 50th to the 84th takes one standard deviation (0.95 kg here); moving from the 97th to the 99th takes much less than that. So a jump between high percentiles can mean a few grams.
- The curve matters more than the dot. Paediatricians watch whether a baby follows roughly the same line over time. Crossing two major centile lines downwards is what prompts a closer look.
Average head circumference by month (WHO median)
| Age | Boys | Girls |
|---|---|---|
| Birth | 34.5 cm | 33.9 cm |
| 1 mo | 37.2 cm | 36.5 cm |
| 2 mo | 39.1 cm | 38.3 cm |
| 3 mo | 40.5 cm | 39.5 cm |
| 4 mo | 41.6 cm | 40.6 cm |
| 6 mo | 43.3 cm | 42.2 cm |
| 9 mo | 45.0 cm | 43.8 cm |
| 12 mo | 46.1 cm | 44.9 cm |
| 18 mo | 47.4 cm | 46.2 cm |
| 2 yrs | 48.3 cm | 47.2 cm |
Frequently asked questions
Why do doctors measure head circumference?
Head growth reflects brain growth, which is fastest in the first two years. Measuring it at each check-up helps spot rare conditions early, such as microcephaly (very small) or hydrocephalus (head growing unusually fast).
What is a normal head circumference for a baby?
At birth the WHO median is 34.5 cm for boys and 33.9 cm for girls. By 12 months it is about 46.1 cm and 45.0 cm. Anything between the 3rd and 97th percentile is considered normal, and big heads often run in families.
How do I measure head circumference?
Use a flexible, non-stretchy tape. Wrap it around the widest part of the head: just above the eyebrows and ears and around the most prominent part at the back. Take three measurements and use the largest.
Data: WHO Child Growth Standards (2006) LMS tables, birth to 5 years; CDC 2000 growth charts LMS tables (cdc.gov/growthcharts), birth to 20 years. Percentiles are computed with the LMS method: z = ((X/M)^L − 1) / (L·S). The CDC recommends the WHO standards for all US children under 2.