µ vs μ: the micro sign is not the Greek letter mu
The micro sign is µ (U+00B5), the unit prefix meaning one millionth. The Greek small letter mu is μ (U+03BC). They look identical in most fonts and are different characters, so a search for one does not find the other.
Last checked 2026-08-05.
At a glance
| Character | Code point | Name | HTML | When it is correct |
|---|---|---|---|---|
| µ | U+00B5 | Micro Sign | µ | Unit prefixes: micrometres, microseconds, microlitres, microfarads. |
| μ | U+03BC | Greek Small Letter Mu | μ | Mathematics, physics notation and any actual Greek text. |
| u | U+0075 | Latin Small Letter U | u | The ASCII stand-in. Acceptable in filenames and code, wrong in prose. |
Why there are two of them at all
The micro sign exists for compatibility. It was present in earlier single-byte character sets, notably Latin-1, and Unicode encoded those sets position by position so that existing documents would survive conversion. The Greek letter came from the Greek block, which was encoded separately.
So Unicode ended up with two code points that draw the same glyph in nearly every font. It has said since that new text should prefer the Greek letter, while also keeping the micro sign valid because millions of existing documents contain it.
Normalisation quietly turns one into the other
Unicode defines four normalisation forms. Two of them, NFKC and NFKD, apply compatibility mappings, and one of those mappings converts the micro sign into the Greek small letter mu. The two decomposed forms, NFC and NFD, leave it alone.
This matters more than it sounds. Databases, search indexes, filename handling and identifier comparison often normalise text before storing or comparing it, and which form they use is rarely documented in the place you would look. The practical consequence is that text you saved with a micro sign can come back with a Greek mu, or two strings you expected to differ can compare equal.
If you are writing code that has to distinguish them, do not rely on a comparison after normalisation. Check the code point.
Which one should you actually type?
For a unit prefix, the micro sign is the semantically precise character and the one the compatibility mapping was created for. For mathematics, physics notation such as the coefficient of friction, and Greek text, use the Greek letter.
In practice the distinction rarely changes what a reader sees, and it frequently changes what a machine finds. If you are producing text that will be searched, indexed or compared, pick one deliberately and apply it consistently across the document rather than typing whichever one your keyboard offered that day.
The same problem in three other places
This is not a one-off. The ohm sign U+2126 and the Greek capital omega U+03A9 have the same relationship, as do the kelvin sign U+212A and the Latin capital K, and the angstrom sign U+212B and the letter A with a ring above. In every case the compatibility character exists for round-tripping older encodings, and in every case normalisation maps it onto the ordinary letter.
The rule that falls out of it is the useful part: when two characters look identical, assume they are different code points until you have checked, and check with a tool rather than with your eyes.
Frequently asked questions
Which one do I use for micrometres?
The micro sign U+00B5 is the semantically correct character for a unit prefix. Be aware that some normalisation steps rewrite it into the Greek mu.
Why does my search not find the character I typed?
Almost certainly because the document has the other one. They look the same and compare as different characters unless the search normalises.
Are there other pairs like this?
Yes. The ohm sign against the Greek capital omega, the kelvin sign against the letter K, and the angstrom sign against the letter A with a ring above all work the same way.