How do I measure a drill bit?

To measure the diameter of a drill bit, you have a number of options. If the machine shop has a set of gauges (a sheet with holes of labeled sizes), you can try twisting the drill bit through each hole until it fits snugly–that is your size. If you don’t have gauges, you can use dial calipers around the diameter (not at the tip). Lastly, you could drill a test hole on a piece of scrap metal and use calipers or a gage pin to determine the size.