(Since there is no "eye roll smiley")
Threads are called out by their diameter, (Say #6 or 6mm), and by threads per inch, or pitch. (32, 24, 20 are common)
So, 6-32, or 1/4-20. I don't think ratio has anything to do with it. There are common sizes for silencers and flash suppressors, from what I've seen, though. I don't remember them, you'll have to look it up.
Also, on a CNC lathe, you can call out ANY numbers and make it into a thread. There are some weird ones......