Twist drills come in a variety of sizes and available in both inch and metric systems. As a rule drills are marked on the shank noting their size. Before using a drill for the first time, however, it should be measured to verify that a size has not been modified. Standard 118 degree drills are often used for modification.

With today's manufacturing technologies it would be the difficult to say what are the limits to how small or how large a twist drill could be made. American Standard drills have been grouped into three sets and designated as:

Fractional set available from 1/64 (.0156) to 3 1/2 (3.500) inches. (above)

Numbers drills (also know as wire gage drills) ranging from #1 (.228) to #90 (.0059). (left)

Letter system that range from "A" (.234) to "Z" (.413). (below)