Well, it's still pi in there, though. Sometimes there's a two, depending on if you start with radius or diameter. But then not for the other. Or just use diameter in calculating circumference, and radius in calculating area, and then you don't need a multiplying (or dividing) factor of 2 in either.
In the case of circumference, it's diameter (or 2 * radius):
Circumference of a circle = pi * diameter
(which also = pi * (radius * 2))
In the case of area, it's pi and radius:
Area of a circle = pi * (radius)^2
(which also = pi * (diameter/2)^2)
