## Find the Circumference of a Circle Given the Diameter or Radius

Now that you know about the relationship between the diameter of a circle and the circumference of the circle, we can work on figuring out the circumference using a formula and pi.

To figure out the circumference of the circle, we multiply the diameter of the circle times pi or 3.14.

$C = d\pi$

Remember, whenever you see the symbol for pi, you substitute 3.14 in when multiplying.

Example 1

Find the circumference.

The diameter of the circle is 6 inches. We can substitute this given information into our formula and solve for the circumference of the circle.

$C & = d\pi\\ C & = 6(3.14)\\ C & = 18.84 \ inches$

What about if we have been given the radius and not the diameter?

Example 2

Find the circumference.

Remember that the radius is one-half of the diameter. You can solve this problem in two ways.

1. Double the radius right away and then use the formula for diameter to find the circumference.
2. Use this formula when you have been given the radius of the circle.

$C = 2\pi r$

Let’s use the formula to find the circumference of the circle.

$C & = 2(3.14)(4)\\ C & = 3.14(8)\\ C & = 25.12 \ cm$