Pi
Pi (Π, π) is the 16th letter of the Greek alphabet, said like “pie”. In maths the small π is the ratio of a circle’s circumference to its diameter, 3.14159…, and the capital Π is the product sign.
Easily confused with
How to read it
Dictionary.com and the Collins English Dictionary both give the pronunciation /paɪ/, the same as “pie”. Collins describes pi as a consonant, transliterated as p.
Also searched as: pi letter, pi sign, pi symbol, π, Π, π meaning, π symbol.
Capital and small forms
| Form | Character | Code point | Unicode name |
|---|---|---|---|
| Capital | Π | U+03A0 | GREEK CAPITAL LETTER PI |
| Small | π | U+03C0 | GREEK SMALL LETTER PI |
| Variant (“omega pi”) | ϖ | U+03D6 | GREEK PI SYMBOL |
| Product sign | ∏ | U+220F | N-ARY PRODUCT |
Unicode describes ϖ as a technical symbol, a variant of pi that looks like Omega; NIST’s guide to the SI lists it as a second italic form of π. The product sign ∏ is a separate character from the letter Π, as the summation sign ∑ is from Sigma.
Pi in maths
The small π is the ratio of the circumference of a circle to its diameter, 3.14159 26535 89793 23846… (NIST DLMF §3.12); Unicode’s own chart notes it as the “math constant 3.141592...”. The capital shape, as the product sign ∏, stands for a product of terms, as ∑ does for a sum. In number theory π(x) is a function, not the constant: the number of primes not exceeding x (DLMF §27.2).
In AI and machine learning
In reinforcement learning π is a policy, the rule an agent uses to decide what actions to take. OpenAI’s Spinning Up guide writes a stochastic policy as π and a deterministic one as μ.
In quantum physics
The reduced Planck constant ħ is Planck’s constant divided by 2π: Unicode names the character ℏ “PLANCK CONSTANT OVER TWO PI” (see Psi). In particle physics π is the pion, a meson: OpenStax lists π⁺ and π⁻ at 139.6 MeV, and the neutral π⁰ is its own antiparticle.
Write in Greek letters
stringmash.comGreek alphabet translatorWrite English in Greek letters by sound, or turn Greek back into Latin letters as Greek passports do.Read next
- Next in Greek lettersRhoRho (Ρ, ρ) is the 17th letter of the Greek alphabet, said “roh” and written r in Latin letters. In science ρ is the symbol for density, and in statistics for the population correlation coefficient.
- In Math symbolsApproximately equal sign (≈)The approximately equal sign (≈) means the two sides are close in value but not exactly equal. π ≈ 3.14 is read “pi is approximately 3.14”.
More greek letters
The same shape elsewhere
Browse symbols drawn with letters and numbers.
Sources
- pi (Dictionary.com entry), Dictionary.com Unabridged; Collins English Dictionary, Dictionary.com, Entry, American and British (Collins) senses
- Greek and Coptic: Range 0370–03FF (code chart), The Unicode Standard, Version 18.0, Unicode Consortium, Greek letters; variant letterforms
- Mathematical Operators: Range 2200–22FF (code chart), The Unicode Standard, Version 18.0, Unicode Consortium, N-ary operators
- Letterlike Symbols: Range 2100–214F (code chart), The Unicode Standard, Version 18.0, Unicode Consortium, Letterlike symbols
- NIST Digital Library of Mathematical Functions, §3.12 Mathematical Constants, DLMF 3.12, National Institute of Standards and Technology, Eq. 3.12.1
- NIST Digital Library of Mathematical Functions, §27.2 Functions (number theory), DLMF 27.2, National Institute of Standards and Technology, §27.2(i)
- Guide for the Use of the International System of Units (SI), NIST Special Publication 811 (2008), National Institute of Standards and Technology, Table 13
- Spinning Up in Deep RL, Part 1: Key Concepts in RL, OpenAI, Policies
- University Physics Volume 3, 11.1 Introduction to Particle Physics, OpenStax (Rice University), Hadrons; Table 11.1
Researched, edited and approved by Rory Hansen before publishing. Last reviewed September 30, 2026. How we research
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