Happy Pi Day! The number pi, or π, is a ratio equal to the circumference of a circle divided by the circle’s diameter. That comes out to about 3.14159. This is true for circles of all sizes, from pennies to pizzas to planets. Pi is an irrational number. The exact value of pi is impossible to write out because the string of digits after the decimal place goes on forever without repeating. Pi’s value is often rounded to 3.14. That’s why Pi Day is celebrated on March 14, or 3/14. It’s also why one