A micrometre is a unit of length, not a tool. It is one millionth of a metre. Put another way: one millimetre holds a thousand micrometres, and one metre holds a million. Scientists and people who make very small parts use it for things too small to think about in millimetres. A human hair is often about 50 to 100 micrometres across. Many bacteria are only a few micrometres long. The symbol is µm. American spelling often writes this unit as micrometer, which is also the name of the measuring tool, so you have to look at the sentence. Older English, and some scientists still, say micron for the same size. You will not hear this word much in the street; it belongs to science and making things.

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