Reusable plastic ring sizer makes it super easy to find your ring size.
It looks like a zip tie, and works like a belt to easily find your US ring size in whole and half sizes from size 1 to size 17 1/2 though using the unmarked lines at the end, it actually sizes to a US size 19 - and yes I can make many (not all) of my rings that big (and bigger).
The ring sizer is 5mm wide, which is roughly the bottom band width of about 90% of my rings' bands. If you are looking to purchase a very wide band ring, you should purchase 2 sizers to get an accurate measurement. The wider the band, the bigger the ring needs to be to be comfortable.