All kinds of calculators and stuff on line. I have apps on my phone and never use them, either. I use the attached chart to tell me the size I need to turn the work to for each class I need and the thread pitch range I need to hit when threading. Everything I need for any class of fit I commonly see in the hobby shop is here, at a glance. You can't get more simple than this chart. It is accurate, complete and quick to use.
So, you pick the tolerance class you need, like class 2a (normal fit external thread like any hardware store bolt), and you turn the work piece to the size range called out for that thread. For example, if you chose to turn a class 2a 1/4-20 thread then you would turn the work piece to within the max and min major diameter (0.2489 - 0.2408" OD). As long as your work piece OD is within this range, the thread fit will be adequate for a class 2 fit. Then you chamfer that work piece, set up your lathe to cut the thread pitch you need (1/4-20 in our case) and cut your threads. There are formulas to estimate total depth of cut; look them up. As you get within 0.010" of max estimated depth begin checking your thread pitch. You can use a mating nut or work piece or you can use your thread micrometer. The chart will tell you that you need to cut until your thread pitch falls within a range and for our 1/4-20 thread, that range is 0.2164 - 0.2127"; cut until your direct reading thread mic gets you inside this range and you're done.
Always clean the thread before taking a reading. Use a wire brush to knock off debris and burrs off of crowns and blow off debris before using your thread mic. High end thread mic anvils are hardened and ground; not so sure about imports so be careful with them.
I can't make it any simpler than this. Hope it helps.