Radius is half of Diameter, therefore you would multiply Diameter by Pi.
You can also find the circumference by doing radius times 2 times Pi. So the formula for circumference is r*2*pi or radius times 2 times Pi
The proper answer, to all those who see this is as follows:
r = radius
π = I hope your joking... But 3.14or pi.
Circumference of a circle is simply pi times the diameter, or 2pi times the radius. Proving this requires some introductory calculus though.
pi times the diameter. That's it, that's the circumference. If you only have the radius, you need to GET the diameter by multiplying the radius by 2. Hence 2rpi
2xr = diameter
diamater x pi = circumference
if the object is a circle or sphere, the cirsunmference is pi (3.1415927) multiplied by the diameter(distance across circle through center point) voila!