Formulas for Circles

The perimeter (outside, or distance around) a circle

• circumference

• area

The space inside a circle

• circumference

• area

C =

• circumference

• area

A =

• circumference

• area

The distance across a circle.

• diameter

• radius

The distance from the center to the outside of a circle.

• diameter

• radius

diameter =

• radius x radius

• radius x 2

• $\frac{1}{2}$ radius

radius =

• 2 x diameter

• diameter x diameter

• $\frac{1}{2}$ diameter

Pi ( $\pi$  )  

• 3.14

•  $r^2$   

d =

• diameter

• radius