Did you know that one piano will use more than a dozen different sizes of plain string?
In this picture, you can see the string sizes of 18, 19, and 20 marked on the plate behind the strings, as well as the half sizes between those. Many modern piano manufacturers indicate the string sizes in this way to make re-stringing easier and faster.
Piano wire (string) varies in sizes from 12 to 25 commonly, including half sizes up through 18 or 20. Unlike electrical wire or other wiring gauges you may be familiar with, piano wire numbers increase with size (that is, 16 gauge would be larger than 14 gauge).
Also, piano wire sizing does not work the same way in the bass section, where strings are copper-wound to increase the mass (and lower the pitch). Bass strings are always custom-sized to the piano, which is also what makes them more expensive to replace.
If you’d like to read more about copper-wound strings, check out these recent posts: