A micrometer is a precision measuring instrument, used in engineering and machining, that measures small distances or thicknesses with high accuracy using a calibrated screw, often called a micrometer screw gauge and can also refer to the unit of length (µm) equal to one-millionth of a meter. As a tool, it typically has a frame, anvil and a spindle that moves with a fine screw to take measurements, providing more precise readings than calipers.