Major jewelers offer printable ring sizers on their websites, such as Lumera Diamonds. The printable ring sizer available through Lumera Diamonds offers three options for measuring the size of a ring or finger.
The most accurate option to find a ring size is to use the measuring tape cut-out. This is wrapped around the finger that the ring will be on, and it gives the exact ring size instead of the finger circumference measurement. The next best option is to simply use a ring that already fits and find the circle that lines up with the ring exactly, and the other option is to measure the finger with a string and then convert that measurement using the chart provided to figure out the ring size.