Using a flexible tape measure (one made of plastic or cloth).
Measure around you head just above the ears not too tightly as a hat must never be too tight, measure a couple of times to be sure …
I have developed my own sizing over time
HEAD CIRCUMFERENCE
|
SIZE
|
21.5″
|
Small
|
22.5″
|
Medium
|
23.5″
|
Large
|
24.5″
|
Extra Large
|