# Student Taught Lesson: Circumference

# Step 1: Pi

Pi (or **π)** is the ratio of a circle's circumference to its diameter. Pi is a very long decimal, and it is usually rounded to 3.14.

# Step 2: Diameter to Radius

The equation for circumference uses radius. Some circles, though, only show diameter, the length across the center of the circle. The way to find the radius of a circle is very easy. For example:

The diameter of this circle is 12 cm. R = D/2, so dividing 12 by 2 gets us the radius.The radius is half of the diameter, so the radius for this circle is 6 cm.

# Step 3: Finding Circumference

The equation for circumference is 2**π**r. To find the circumference of the circle above, we have to use this equation.

C = 2**π**r **π = 3.14**

C = 2 x 3.14 x 6

C = 37.68 cm

The circumference of this circle is 37.68 cm.

1. r = 10

2. r = 7

3. r = 12