How to figure out my hat size for a true fitted 59 fifty hat (flat rimmed)?

i want to get my mom a true fitted 59 fifty mets hat (flat rimmed) for her b-day in a few days, so i took a piece of string and put it around her head, and where the string met i marked it, and measured it. it measured about 21 and 1/2 inches. what would be her hat size?