While very few of us will be doing projects that require measurements down to the 100ths of an inch, a set of
cheap value priced calipers is something I think everyone should have. I got the set pictured at (I think) Harbor Freight for less that $5. Amazon sells them as well, a 3 pack for less than $12. I would stay away from the ones with the dial or digital readout, unless you know far more about them than I am covering in this post.
The ones that are just a one piece sliding on another have nothing that can go wrong, other than physically breaking or the markings wearing off. These will measure inside diameter, outside diameter and depth. In this example I need to know what size hole saw to use to mount this gate valve in a bucket that will hold honey. It is maybe 1/64″ of an inch smaller than 1-3/4″, so I will be using a 1-3/4″ hole saw and let the gasket take care of the rest.