21 month old head circumference
On the WHO growth standard, the average (median) head circumference of a 21-month-old baby boy is 47.8 cm (18.8 in) and of a girl 46.7 cm (18.4 in). The usual range, from the 3rd to the 97th percentile, is 45.3–50.4 cm for boys and 44.1–49.4 cm for girls.
Head circumference percentiles at 21 months (WHO)
The median (50th percentile) is the average head size for this age.
| 3rd | 15th | Median | 85th | 97th | |
|---|---|---|---|---|---|
| Boys (cm) | 45.3 | 46.4 | 47.8 | 49.2 | 50.4 |
| Boys (in) | 17.8 | 18.3 | 18.8 | 19.4 | 19.8 |
| Girls (cm) | 44.1 | 45.3 | 46.7 | 48.2 | 49.4 |
| Girls (in) | 17.4 | 17.8 | 18.4 | 19.0 | 19.4 |
Check your baby’s head circumference percentile
Enter the exact date of birth for a precise result, or use the centimetre table below.
Enter a date of birth and at least one measurement to see the percentile.
What percentile is my 21 month old’s head circumference?
WHO percentile for each head circumference, in half centimetres, at exactly 21 months. A few days older or younger shifts it slightly.
| Head | Boy | Girl |
|---|---|---|
| 44.0 cm (17.3 in) | below 1st | 3rd |
| 44.5 cm (17.5 in) | below 1st | 6th |
| 45.0 cm (17.7 in) | 2nd | 11th |
| 45.5 cm (17.9 in) | 4th | 19th |
| 46.0 cm (18.1 in) | 9th | 30th |
| 46.5 cm (18.3 in) | 16th | 43rd |
| 47.0 cm (18.5 in) | 27th | 57th |
| 47.5 cm (18.7 in) | 40th | 71st |
| 48.0 cm (18.9 in) | 55th | 82nd |
| 48.5 cm (19.1 in) | 69th | 90th |
| 49.0 cm (19.3 in) | 81st | 95th |
| 49.5 cm (19.5 in) | 89th | 98th |
| 50.0 cm (19.7 in) | 95th | 99th |
| 50.5 cm (19.9 in) | 98th | above 99th |
Common questions
What is the average head circumference of a 21-month-old baby?
On the WHO growth standard, the average (median) head circumference of a 21-month-old baby boy is 47.8 cm (18.8 in) and of a girl 46.7 cm (18.4 in). The usual range, from the 3rd to the 97th percentile, is 45.3–50.4 cm for boys and 44.1–49.4 cm for girls.
What percentile is a 47 cm head circumference at 21 months?
On the WHO chart, 47 cm (18.5 in) at exactly 21 months is about the 27th percentile for a boy and the 57th percentile for a girl. The table on this page gives the percentile for every half centimetre in the usual range.
How much does head circumference grow around 21 months?
Going by the WHO medians, about 0.1 cm for boys and 0.2 cm for girls in the month leading up to 21 months. Head growth is fastest in the first months and slows during the second year.
Is my 21-month-old's head too big or too small?
A head circumference between the 3rd and the 97th percentile is within the usual range, and head size often runs in families. Pediatricians look most at the trend: talk to yours if the measurement is below the 3rd or above the 97th percentile, or if the curve crosses two of the main percentile lines between visits.
Also: 21 month old baby weight · 21 month old baby length · head circumference chart by month · about head circumference
Source: WHO Child Growth Standards, head circumference-for-age, percentiles computed from the WHO LMS tables at 21 completed months. Measure around the widest part of the head, above the eyebrows and ears. For information only; not medical advice.