SAE are measured in threads per inch. And the difference between the sides is far enough apart that you don't need to be ridiculously accurate. If you have a quarter inch of thread available, you just count the number of turns,in that 1/4 inch, and multiply by 4. This works for bolts.
Tubing is measured a little differently.
They only put one type of thread on each size of braketube, so in reality you only need to know the size of the tube that you are working with, and you will automatically get the right fitting by specifying the tube size.
Metric is a different deal,(see below) and I can't imagine you will encounter that on a KH system.
as also implied below, 1 inch = 25.4mm
And 3/16 tubing is 4.77mm, but in your case; 4.77mm is 3/16 tubing,lol..
