Average Head Circumference for a 21-Month-Old Boy

The average (50th-percentile) head circumference for a 21-month-old boy is 47.8 cm (18.8 in), based on the WHO Child Growth Standards. Across the healthy range, most boys this age have a head circumference between 45.3 cm (17.8 in) and 50.4 cm (19.8 in). (measured around the largest part of the head)

What's typical at this age

Head growth is very gradual now, and the anterior fontanelle closes in most toddlers between 12 and 24 months. By age two the brain has reached about 80% of its adult size. Between month 21 and month 22, the median boy's head circumference grows about 0.1 cm.

A percentile compares your boy to other boys the same age — the 50th is simply the middle. Being above or below it is usually completely normal; a baby who has always tracked the 15th or 85th percentile is typically growing exactly as they should. What pediatricians watch for is a sudden change in the pattern, not the number itself.

Percentiles calculated from the WHO Child Growth Standards (2006). For information only — not medical advice. If you're concerned about your child's growth, speak to your pediatrician or health visitor. Full disclaimer.