How to Measure Hat Size. ... Note that most people’s hat measurement falls between 51 and 64 centimeters, or 20 to 25 inches. 5. Refer to the Brand’s Sizing Guide .

Hey Ray: Using Pi to measure hat size 02:12. There is Pi all around us, in ways you may not have seen before. If you remember from school, Pi is the ratio of a circle's circumference to its diameter.