Adult hat sizes are usually determined as follows:
Hat Size
Circumference
Small
Size 6-3/4
21"
Size 6-7/8
21-1/2"
Medium
Size 7
22"
Size 7-1/8
22-1/2"
Large
Size 7-1/4
22-3/4"
Size 7-3/8
23"
Extra large
Size 7-1/2
23-1/2"
Size 7-5/8
24"