Ring Sizer
Your ring should fit you perfectly. After all, it's made for you.
Ensure fit with this ring sizer. Works like a belt-- slide it on and pull the end to adjust. The sizer should fit the same as a ring, slipping over the knuckle comfortably.
Measures 1-17 sizes and half sizes. USA sizes (see other country conversions here).
Sample photo one shows size 7.5.