If you regularly measure fingers for ring size or you really don't trust your own accuracy when it comes to the string and measuring tap method, there are dedicated tools you can use to measure your ring size.
The first, and the easiest tool to use, is simply a collection of 27 stainless steel rings, each of which has its size marked on the link. The rings are connected to a wire loop so they stay together. Once you figure out which ring fits you the best, you have your size.
Another option is to get a ring-sizing tool called a mandrel. A mandrel is a long dowel with an increasing girth and ring sizes marked along its expanding length. Just take whichever ring you already own that's the most comfortable and slide it onto the tool to find your size. You'll know the size by the marking on the mandrel where it stops.
Buy the KUUQA Ring Sizer on Amazon for $6.89 (originally $9.99)
Buy the Rosenthal Ring Collection Ring Sizer Mandrel on Amazon for $6.70