Most drill bits are stamped with a size on the shaft of the bit. It is not unusual, however, for this indicator to wear off with use. Bits can also wear down with use, effectively changing the size of the bit. It is not always necessary to use a bit that is precise to a thousandth of an inch, but you may find it is required from time to time, depending on the project.
Hold the drill bit by the shaft so that the tip of the bit is facing upward. Hold it with your fingers so that you can easily rotate it.
Open the jaws of the calipers so that they are wider than the drill bit. Position the tip of the drill bit in between the jaws.
Close the jaws of the caliper on the tip of the drill bit, softly. Rotate the drill bit with your fingers. Do not maintain pressure with the calipers. Allow them to slide open as the drill rotates so that you can locate the widest point of the drill bit.
Apply light pressure to close the jaws at the widest point of the drill bit circumference. When rotating the bit the jaws may open slightly larger than the actual dimension of the bit.
Record the reading from the calipers. This is the finished size of the hole that will be produced by the drill bit.
If less accuracy is required, using a drill gauge is an easy way to determine approximate sizes of drill bits.
If you are not sure if the drill bit is a metric, fractional or numbered size, it may be difficult to get an accurate size with a drill gauge as the differences in a fractional, numbered or metric bit can be marginal.