In geometry, the terms circumference and diameter refer to the length of specific parts of a circle. They are two different measurements of length, but they share a special mathematical relationship with the constant pi.

## Diameter

The diameter is the length, or distance, across the circle at its widest point, passing through the center. Another related measurement, the radius, is a line that goes from the center to the circle's edge. The diameter is equal to 2 times the radius. (A line that goes across the circle, but not at its widest point, is called a chord.)

## Circumference

The circumference is the perimeter, or distance around the circle. Imagine wrapping a string all the way around a circle. Now imagine removing the string and pulling it out into a straight line. If you were to measure this string, that length is the circumference of your circle.

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## Pi

The quantity pi is a mathematical constant defined as the ratio of a circle's circumference to its diameter. This ratio is always the same. If you divide the circumference of any circle by its diameter, you always get pi. Mathematicians use the number 3.14 when using pi in calculations.

## Relationship Between Diameter and Circumference

If you know a circle's diameter, you can calculate its circumference with this equation: Circumference = diameter times pi (3.14).