The average human head is 6 to 7 inches wide and 8 to 9 inches long. The average circumference is 21 to 23 inches. Males generally have a slightly larger head than females.
As a proportion of the total body, a baby's head is approximately one-quarter of the total length. The ratio changes to one-seventh of the total length in an adult. The circumference of the head also changes as humans mature, expanding rapidly in the first few years, then slowing when an adult reaches the age of 20. Besides changing in size, the human skull also hardens, with bones fusing together as the size changes.