If µ and μ look identical on your screen, that is normal. Many fonts draw them the same way. Unicode still stores them as two different characters: MICRO SIGN and GREEK SMALL LETTER MU.
The confusion becomes important in measurement notation because the SI prefix micro uses a mu-shaped symbol for 10−6. SI defines the prefix and its meaning; Unicode decides which characters can store that shape. Those are related questions, but not the same one.
Two mu-shaped characters
| Character | Unicode name | Code point | What it is |
|---|---|---|---|
| µ | MICRO SIGN | U+00B5 | Compatibility character inherited from legacy encodings |
| μ | GREEK SMALL LETTER MU | U+03BC | Greek alphabet letter |
Why both characters exist
U+00B5 MICRO SIGN comes from older character sets, including Latin-1, that provided a dedicated micro character without encoding the full Greek alphabet. Unicode also encodes the actual Greek letter mu separately at U+03BC.
Keeping both code points lets Unicode preserve older data faithfully. It also means that two strings can look identical while containing different characters.
Unicode’s position
Unicode does not make U+00B5 and U+03BC canonically equivalent. U+00B5 instead has a compatibility mapping to U+03BC. The Unicode Core Specification notes that both characters are commonly used as a micro sign, while Unicode Technical Report #25 identifies U+03BC as the preferred character in a Unicode mathematical context.
That is character-encoding guidance, not an SI requirement. The SI defines the prefix micro and its symbol; it does not specify a Unicode code point for storing that symbol.
SI defines the prefix, not the code point
The SI prefix micro has the symbol µ and the factor 10−6. The SI Brochure presents the prefix symbol visually; it is a measurement standard, not a Unicode code-point specification.
In digital text, the mu-shaped prefix may therefore appear as U+00B5 MICRO SIGN or U+03BC GREEK SMALL LETTER MU. U+03BC aligns with Unicode’s preferred-character guidance for modern Unicode mathematics, while U+00B5 remains common in scientific data and legacy-compatible systems. Neither code point changes the prefix factor.
What normalization does to µ
U+00B5 has a compatibility decomposition to U+03BC. Applying NFKC maps µ to μ, whereas NFC leaves the distinction in place because Unicode does not define the pair as canonically equivalent.
This is useful for search and matching, but it changes the original code point. A system that needs both reliable matching and source fidelity can preserve the raw text while indexing a normalized form separately.
Where mixed encodings cause trouble
Consider two values that appear to start with the same symbol: µm and μm. One may begin with U+00B5 and the other with U+03BC. A literal string comparison can treat them as different even though a reader interprets both as the micro prefix followed by metre.
The same issue can affect search, grouping, validation, identifiers, and unit parsers. If visually identical values fail to match, checking the first code point is more useful than comparing the glyphs.
How micro joins a unit symbol
When micro acts as an SI prefix, it becomes part of the unit token: μm, μs, μF. Write a value as 25 μm—the gap comes before the complete unit, never between the prefix and the unit it modifies.
If an ASCII-only system cannot carry the mu-shaped character, um may appear as a practical fallback, but it is different text and is not the SI prefix symbol. The related micrometre symbol vs um guide looks at that case directly.
How to inspect the actual character
Do not rely on the font. Many typefaces deliberately render U+00B5 and U+03BC the same or nearly the same. Inspect the code point with a Unicode-aware editor, character inspector, or diagnostic tool.
Context tells you what the mark probably means, but not necessarily which character is stored. In a value such as 50 µm, the measurement intent is clear even if the underlying mu-shaped code point is not.
A tolerant parser should accept both
Even if newly authored Unicode text uses μ, real-world scientific data still contains large amounts of U+00B5 µ. A parser that accepts only one code point can reject valid-looking measurements simply because they came from a different software stack or an older source.
In software, it is usually better to accept both forms on input and apply one documented normalization policy internally. If the original spelling has archival value, keep it alongside the normalized value rather than asking the glyph to do work it cannot do.
Choosing a representation
For newly authored text, follow the encoding policy of the system you are publishing into. U+03BC fits Unicode’s preferred-character guidance; U+00B5 still appears widely in scientific and legacy data and may need to be preserved when source fidelity matters.
For readers, both glyphs usually communicate the same micro prefix in context. For software, the safer approach is deliberate: recognize both, normalize intentionally, and keep SI semantics separate from Unicode encoding.