5 Helpful Crochet Size Charts from Starting Chain - includes links to hat sizing chart, blanket sizing chart, mitten pattern, baby clothes sizing chart, and baby bootie sizing chart. I have been looking for a hat sizing chart.
Crochet Hat Sizing Table - No circumference! so, this is for crochet, so you can measure starting at the brim, cause you crochet top - down. I wonder about knit though, when you knit bottom - up ? There's also a chart in centimeters.