Circumference of a circle

The circumference of a circle is the length of the circle itself (or of the boundary of the disc). Let the radius be r. Then the circumference of the circle, which we denote C, is

$$C=2\cdot \pi \cdot r$$

The symbol π stands for pi. Its approximate value is 3.14, but we usually leave the answer as a multiple of pi. If we know the diameter d of the circle instead of the radius, we use this formula:

$$C=\pi \cdot d$$

The radius of the circle below is r = 3, so by the first formula the circumference is

$$C=2\cdot \pi \cdot 3=6\pi$$