Using the Pi Symbol
Pi is the sixteenth letter of the Greek alphabet, and since William Jones adopted it in 1706 (and Euler popularised it), the lower-case form has named the ratio of a circle's circumference to its diameter. That number, 3.14159…, is irrational and turns up far beyond circles: in the normal distribution, in Fourier series, in Euler's identity e^(iπ) + 1 = 0. The letter itself is what you copy from this page; the value is something a calculator supplies.
Formulas that use it
- Circumference: C = 2πr, or πd.
- Area of a circle: A = πr².
- Volume of a sphere: V = (4/3)πr³.
- Radians: a full turn is 2π, a right angle is π/2.
In all of these π is set in upright or italic type depending on the house style; ISO recommends upright for constants, but most textbooks leave it italic like any other Greek variable. There is no space between π and a following variable (πr², not π r²), and when π multiplies a plain number the number usually goes first: 2π, not π2.
Capital Π and the product sign
The capital letter Π is used in older texts for products, but modern notation prefers the dedicated operator ∏, which is larger and takes limits above and below. The variant ϖ, sometimes called omega pi because of its shape, is a separate character used when a physicist needs a second pi-like symbol, for example for the longitude of periapsis in orbital mechanics. Keep to the plain π in ordinary writing and reserve the others for their special jobs.
Typing π
macOS makes it easy: Option + P. Windows has no Latin-1 code, but the DOS-era Alt + 227 produces π in many programs, and Word converts 03C0 to π after Alt + X. Gboard on Android puts it on the second symbols page beside √ and ÷, and iPhone users can either add the Greek keyboard, where π lives on the P key, or set up a text replacement. Because it is a Greek letter rather than a maths operator, it appears in almost every font on every device, including the ones used inside games and chat apps.
Pi Day and pop culture
14 March, written 3/14 in the American date order, is Pi Day, and 22 July (22/7) is Pi Approximation Day. Both fill social feeds with π in captions, pie recipes and "π ≈ 3.14" jokes, which is where the approximately equal sign earns its keep. Maths teachers put it in display names, students use π in usernames on Discord, and the film Life of Pi and Darren Aronofsky's Pi keep the letter in the wider culture. It also pairs well with the square root symbol and the infinity symbol in a maths-themed handle, and it holds up in the small fonts generator when you want the whole bio in tiny type.