Baby and toddler head circumference percentile from the official WHO standards, birth to 60 months, with the z-score, median and the 3rd–97th percentile range.
| Input | Result |
|---|---|
| 12-month-old boy, 46.1 cm | about the 51st percentile |
| 24-month-old boy, 49.6 cm | about the 84th percentile |
| 6-month-old girl, 42.2 cm | about the 50th percentile |
| 36-month-old boy, 49.9 cm | about the 62nd percentile |
A growth percentile compares your child with a reference population of the same age and sex — not with "normal". The 50th percentile is the median, not a target. A healthy child can sit steadily at the 10th percentile for years, and that is a different situation from a child who drops from the 60th to the 10th in six months.
This is why pediatric practice looks at the curve, not the number. A single measurement tells you where a child is; two or three measurements over time tell you how they are growing, which is the thing that actually carries information. Very few children track a constant percentile — most move up or down one band between visits and that is normal.
The reference values are the WHO Child Growth Standards, published by the World Health Organization from its Multicentre Growth Reference Study, which followed healthy, breastfed infants in six countries. WHO releases the standards as LMS parameters — the official machine-readable form of the growth curves:
M is the median (50th percentile), S the coefficient of variation and L the skewness power. Head circumference uses L = 1, so the formula simplifies to z = (X/M − 1) / S. Age is interpolated linearly between WHO's published whole-month rows.
The data embedded in this page are the WHO files themselves (the same tables pediatric clinics plot against), not estimates or re-digitized curves.
Use a flexible, non-stretch tape. Place it just above the eyebrows and the tops of the ears, passing over the most prominent point at the back of the head — this is the position that gives the largest circumference, and it is the WHO measurement standard. Take the measurement two or three times; repeat readings normally differ by up to half a centimetre, and the largest consistent reading is the one to record.
Measure without compressing the hair or skin, and note that a thick hairstyle or a recently applied hat can add millimetres. Clinic visits use the same landmark points with a calibrated tape, which is why a home reading and a clinic reading can legitimately differ by a few percentiles.
The M column of the WHO LMS file is the published 50th percentile, so these are the reference values themselves, read straight from the WHO tables.
| Age | Boys | Girls |
|---|---|---|
| 0 months | 34.46 cm | 33.88 cm |
| 6 months | 43.33 cm | 42.20 cm |
| 12 months | 46.07 cm | 44.90 cm |
| 18 months | 47.37 cm | 46.24 cm |
| 24 months | 48.25 cm | 47.18 cm |
| 30 months | 48.94 cm | 47.93 cm |
| 36 months | 49.46 cm | 48.51 cm |
| 42 months | 49.87 cm | 48.96 cm |
| 48 months | 50.21 cm | 49.33 cm |
| 54 months | 50.49 cm | 49.65 cm |
| 60 months | 50.74 cm | 49.92 cm |
| Age | Correct reference | Where on this site |
|---|---|---|
| 0–2 years | WHO growth standards | This page. The CDC and the American Academy of Pediatrics both recommend WHO charts for under-2s. |
| 2–20 years | CDC 2000 growth charts | Child weight, child height and child BMI calculators. |
Switching charts at age 2 produces a small visible step in the curve, because the two references were built from different populations. Keep a child's series on one chart per age band.
Head size tracks brain growth, which is fastest in the first two years. A head growing steadily along its curve is reassuring; a head that stops crossing percentiles, or accelerates across them, is the signal pediatricians look for.
Use a non-stretch tape placed just above the eyebrows and ears, passing over the most prominent point at the back of the head — the position that gives the largest circumference. Take two or three readings and use the largest consistent one. A difference of half a centimetre between attempts is normal.
Head circumference below the 3rd percentile (microcephaly screening) or above the 97th (macrocephaly screening) are the common cutoffs that prompt evaluation. They are screening thresholds, not diagnoses — many children outside them are perfectly healthy, and family head size matters.
No. Within the normal range, head size percentile carries no meaningful information about intelligence. Only extreme measurements combined with developmental findings are clinically informative.
The WHO head circumference standards run from birth to 60 completed months, which is the range this calculator covers. Head circumference is most informative in the first 24 months, when brain growth is fastest.