Phi (uppercase Φ, lowercase φ or ϕ), pronounced [fī] in modern Greek and as [faɪ] in English, is the 21st letter of the Greek alphabet. In modern Greek, it represents [f], a voiceless labiodental fricative. In Ancient Greek it represented [pʰ], an aspirated voiceless bilabial plosive (from which English ultimately inherits the spelling "ph" in words derived from Greek). In the system of Greek numerals it has a value of 500 (φʹ) or 500,000 (͵φ).

The lower-case letter varphi , (or often its variant, phi ,) is used as a symbol for:

The upper-case letter Φ is used as a symbol for:

The diameter symbol in engineering, , is often incorrectly referred to as "phi". This symbol is used to indicate the diameter of a circular section, for example ⌀14 means the diameter of the circle is 14 units.


In Unicode, there are multiple forms of the phi letter:

  • upper case:
    • U+03A6 GREEK CAPITAL LETTER PHI (Φ): Greek capital letter phi
  • lower case:
    • GREEK SMALL LETTER PHI (φ): letter phi, used in Greek texts.
    • U+03D5 GREEK PHI SYMBOL (ϕ): phi symbol, for mathematical and technical contexts.

In HTML/XHTML, the upper and lower case phi character entity references are Φ (Φ) and φ (φ) respectively.

In LaTeX, the math symbols are Phi (Phi,!), phi (phi ,!), and varphi (varphi,!).

In some browsers (e.g. Internet Explorer 6), the shapes of the U+03C6 GREEK SMALL LETTER PHI (which should be viewed as a curl) and U+03D5 GREEK PHI SYMBOL (which should be viewed as a circle crossed by a slash) are exchanged. Compare these samples to check your browser:

Phi letter: right varphi,!; possible wrong φ
Phi symbol: right phi,!; possible wrong ϕ

See also


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