New Era Cap Sizing guide

To find the correct cap size, measure around the circumference of the head with a flexible tape, where the sweatband will rest, ensuring that the tape measure is neither too loose or too tight. Simply convert using the measurement chart. For example, if the measurement was 22 inches [55.8 cm] then you would be a size 7 etc.

New Era Cap Sizing Guide
More information here on this short video - Youtube