Micro sign µ

• µm (micrometer): in US industry, often called a micron.
• µg (microgram): vitamins, supplements and medications.
• µs (microsecond): timing in electronics and computing.
• µF (microfarad): capacitor values.
• µL (microliter): lab volumes.

The prefix goes directly before the unit with no space, and the number takes a space before the unit: 25 µm, 400 µg.

In handwritten or typed medical orders, µg can be misread as mg, a thousand times larger. US medication-safety guidance from ISMP therefore recommends writing mcg instead of µg.

There are two nearly identical characters:

• µ (U+00B5, micro sign) comes from older Western character sets, which is why it is easy to type on US keyboards.
• μ (U+03BC, Greek small letter mu) is the Greek letter.

Unicode treats the micro sign as a compatibility character: under NFKC normalization it becomes the Greek mu. Both are accepted for the micro prefix, and most fonts draw them the same. Use μ in Greek text and where mu is a variable, such as μ for a population mean in statistics or the coefficient of friction in physics.

When no µ is available, plain text and older systems use u instead: um, ug, uF. That works in file names and code but looks sloppy in published text.

• LaTeX: \mu gives the Greek letter in math mode. For units, the siunitx package handles the prefix: \SI{25}{\micro\meter}. In text, \textmu gives the micro sign.
• Excel: custom number formats can show the unit while the cell stays numeric, for example 0 "µF".
• iPhone and Android: add the Greek keyboard to type μ, or copy µ here and save a text replacement such as “mcr” → µ.
• Search tip: because µ and μ are different characters, search for both when looking through datasheets or lab notes.