A micrometer is a precision measuring tool that can measure extremely small distances, usually to within one thousandth of an inch or one hundredth of a millimeter.

If you tried to measure the thickness of a sheet of paper with a regular ruler, you'd have trouble: the lines are too far apart and the paper is too thin. A micrometer solves this problem. It has a calibrated screw mechanism that closes slowly and precisely around an object, giving you an exact reading of its thickness or diameter. Machinists use micrometers to check whether metal parts meet exact specifications. Scientists use them to measure the width of tiny specimens. Anyone working where precision matters, like engineers building engines or jewelers crafting delicate pieces, relies on micrometers.

The word also refers to a unit of length equal to one millionth of a meter, though scientists more commonly call this a micrometer (symbol: μm). A human hair is about 70 micrometers wide. A red blood cell is about 7 micrometers across. When scientists describe bacteria or examine materials under microscopes, they often measure in micrometers because regular units like inches or centimeters are far too large.