Head and Hat Sizes Chart
There are variations from chart to chart. But below are some useful guides for hat sizes.
Useful link: http://www.ubs.iastate.edu/hat_sizing_chart.html. (Noted in Sep 2024, the link is no longer available.)
Below is a screenshot from this link.
Here is another handy chart, including how to measure head for size. This image below is from Rosehip Hat Studio.
