The formula for circumference is 2(pi)(r), or C = 2(pi)(r)
The diameter will always be TWICE the length of the radius (radius represents r in the formula), which means that in order to find radius from the diameter, you would have to divide the value for the diameter by 2.
So r = 45/2 = 22.5
Now that you have the radius, you can plug it into the equation for circumference, giving us:
C = 2(pi)(22.5) = 45(pi)
The circumference is 45 multiplied by pi. However, if you need EXACT circumference, you would have to use a calculator to multiply pi by 45.
