Capacity numerical

1. Homework Statement :

Taking earth to be a spherical conductor of diameter 12.8x10^3 km.its capacity will be?

3. The Attempt at a Solution :
if earth is to be taken as a conductor than i think following formula shall be used
C=Aeo/d
where A=pie r^2
but how i'll calculate distance?(if the above formulae will be used)

d is the radius of Earth. But the area is not $\pi r^2$. You need to find the outer surface area....what is the surface area of a sphere?