Pi (π) is the ratio of the circumference of a circle to its diameter. The value of pi is 3.1416..., an irrational number.

3.14 and 3 (or ) are often used as approximate values for pi.

Pi is a constant value. That is, the ratio of the circumference to the diameter is the same for all circles.

The drawing below shows the circumference of the circle "stretched out." It is a little more than three diameters in length:

See also circumference, irrational numbers.