Hat Size Chart
Here is a chart that shows average hat measurements
for 7 sizes from Preemie to Large Adult.
Each size is given for a hat with a loose fit, tighter fit and snug fit to make it easier for you to make a hat in just the right size and fit for anyone.
If the hat you plan to knit is intended for somebody else, and asking him or her for a favourite hat or head measurements is out of the question, use this chart as a guideline.