Micro sign (µ) in HTML
The micro sign is the SI prefix for one millionth. Its named reference is µ and its code point is U+00B5, which is a different character from the visually identical Greek small letter mu.
At a glance
- Character
- µ
- Named entity
- µ
- Decimal reference
- µ
- Hex reference
- µ
- Unicode code point
- U+00B5
- Unicode name
- MICRO SIGN
- In the entity reference
- Symbols
In detail
Two characters in Unicode draw the same shape. The micro sign at U+00B5 exists because it was in Latin-1 and Unicode preserved round trip compatibility with that encoding. The Greek small letter mu at U+03BC is the actual Greek letter. They render identically in almost every typeface, and no amount of staring at a screen will tell you which one is in your file.
For units, the recommendation from the Unicode side is to use the Greek letter, since the micro sign is a compatibility character. In practice both are everywhere: keyboards and word processors on some platforms insert one, scientific software the other, and the micro sign is what you get from the named HTML entity. Either will render correctly for a reader.
The problem is machines. Compatibility normalisation, the form usually written NFKC, maps the micro sign onto the Greek letter, so a search index that normalises will treat them as equal and one that does not will treat them as different words. A units column in a database can therefore contain two values that look the same, sort differently and fail an equality check.
The practical rule is to normalise on the way in. Pick one code point for your content, convert at the point of ingest, and never rely on a visual review to catch the other. If you are storing measurements, storing the value and the unit separately in a controlled vocabulary removes the question altogether.
Examples
<p>micro sign: µs (µ)</p>
<p>Greek small mu: μs (μ)</p>const a = "\u00B5s";
const b = "\u03BCs";
a === b; // false
a.normalize("NFKC") === b.normalize("NFKC"); // trueThings that catch people out
- A units filter written against one code point silently misses every record stored with the other, and the mismatch is invisible in any interface that renders both.
- Case conversion does not help. The micro sign uppercases to the Greek capital mu in some implementations, which turns a unit into a letter.
- Copying a value out of a spreadsheet can change which character you get, because the spreadsheet may have normalised the cell on import.
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Related HTML entities
All html entities →Mu (μ) in HTML
The Greek letter mu at U+039C and U+03BC, its entity forms, the separate micro sign it duplicates, and its meanings in statistics and mechanics.
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