What is the average human head circumference?

The average head circumference of an adult male and female is 22.44 and 20.87 inches, respectively. However, averages vary widely depending on the age and ethnicity of the individual.

Forensic anthropologists at the University of Tennessee, Knoxville have recently determined that American head size is on the rise. By studying skulls from the mid-1800s through the mid-1980s, researchers discovered that the average skull "has grown by 200 cubic centimeters, a space equivalent to a tennis ball." Shape and length have also morphed over the last century, and in a disproportionate manner. For example, skull height has grown by nearly 7 percent, yet femur length has only increased 2 percent.