I've found the best way to determine ring size is this handy - dandy app called "Ring Sizer by Jason Withers" it's really accurate and you simply use a ring that already fits you the way that you like.
OR this method is great as well ↴
HOW TO MEASURE YOUR RING SIZE WITH A PRINTABLE SHEET:
Print out this sheet and place your ring on the circles until you find the one that matches up with the inside circumference of your ring.
Regardless of how you decide to go about finding your ring size, I am always here to help and answer questions. Please reach out to me on my "Contact" page via this website.