Hi Jay, Thank you for reaching out to us. This cap that you are asking the question on , adjusts from 7 1/2 to 8 1/2. (2XL - 4XL) To answer your question on inch difference between 7 1/2 to an 8 is an 1.525 inches. 7 1/2 translates to 23.6 inches around and 8 translates to 25.125 inches. Take the inches around your head and divide by 3.14 (pi) to get your actual hat size. Definitely call us or email directly with any measurement questions so that we can assist you in getting the right cap for you. I have a link below for on our website of how to measure your head with sizing chart too. Don't hesitate on how we can assist you.
https://www.bigheadcaps.com/howtomeasureyourhead