A micrometer is a widely used device in mechanical engineering for precisely measuring thickness of blocks, outer and inner diameters of shafts and depths of slots. Micrometers have several advantages over other types of measuring instruments like the Vernier caliper: they are easy to use and their readouts are consistent. There are three types of micrometers based on their application:

• External micrometer
• Internal micrometer
• Depth micrometer

An external micrometer is typically used to measure wires, spheres, shafts and blocks.

An internal micrometer is used to measure the opening of holes, and a depth micrometer typically to measure depths of slots and steps.

The precision of a micrometer is achieved by a mechanical advantage of a fine pitch screw mechanism.

The first ever micrometric screw was invented by William Gascoigne in the 17th century, as an enhancement of the Vernier; it was used in a telescope to measure angular distances between stars. Its adaptation for the measurement of the small dimension was made by Jean-Louis Palmer ; this device is therefore often called palmer in France.

Micrometer is also the U.S. spelling of micrometre, a millionth of a metre or meter. This word is pronounced with stress on the first syllable, while the name of the measuring device is stressed on the second.