What the pi symbol means
π (pi) is the ratio of any circle's circumference to its diameter, about 3.14159. It is an irrational number, so its decimals never end and never repeat. It shows up in the area of a circle, πr², its circumference, 2πr, and in angles, where 180° equals π radians.
The Welsh mathematician William Jones first used π for this constant in 1706, and Leonhard Euler made it popular in the following decades. Pi Day is celebrated on March 14 (3/14), often with a pie.
Capital Π is a different tool: mathematicians use Π, or the dedicated product sign ∏, for multiplying a series of terms, the way Σ adds them. In economics, π often stands for profit or inflation, and in statistics for a population proportion.
π, Π, ∏ and ϖ: what is the difference?
π (U+03C0) is the Greek small letter and the one you want for the number pi. Π (U+03A0) is the capital letter. ∏ (U+220F) is the math product operator, drawn larger, as in ∏ xᵢ. ϖ (U+03D6), called variant pi or pomega, looks like an omega with a bar on top; astronomers use it for the longitude of perihelion.
Math fonts add more options: the italic 𝜋 seen in textbooks, a bold 𝛑 and the double-struck ℼ. Avoid faking π with "n" or "TT", since search engines and screen readers will not understand it.
How to type π
On a Mac, press Option + P for π and Option + Shift + P for ∏. On Windows, hold Alt and type 227 on the number pad. In Word, type 03C0 and press Alt + X, or insert an equation and type \pi.
On Android, Gboard has π on its second symbols page: tap ?123, then =\<. The standard iPhone keyboard has no π, so add the Greek keyboard (the P key types π) or set up a shortcut in Settings > General > Keyboard > Text Replacement.
In LaTeX, use \pi, \Pi, \prod and \varpi. In HTML, use π. In Excel and Google Sheets, =PI() returns pi to 15 digits. For other circle symbols, see the diameter symbol, and for every operator see math symbols.