Hole micrometers are used to measure the internal dimensions of usually cylindrical objects such as bearing cylinders, etc. They can measure diameters with a high degree of accuracy, typically within a few thousandths of an inch or hundredths of a millimeter.

They are divided into bar-type micrometers, 3-contact micrometers, and micrometers with "hammer" features.

The internal bar micrometer consists of a long, thin shaft with a spindle at one end that is inserted into the object being measured and the measuring mechanism at the other end. The shaft rotates to apply pressure inside the object, and the measurement is read from the scale on the outside of the tool.

A 3-point micrometer, also known as a 3-contact micrometer, is a type of micrometer used to measure the diameter of cylindrical objects with high precision.