The Capacitance of a Cylindrical Capacitor equation describes the capacitance of two parallel plates of equal area A and with separation d. We're working under the assumption that the plates are much longer and wider than the distance that separates them. This equation can be derived from Gauss's Law and the relation q = CV 1.
A somewhat abstract representation of a capacitor
Variables
A - the area of each plate
d - the distance between the plates
εrεr - The Dielectric Constant, a.k.a. Relative Permittivity, of the material between the shells. For more info, click here.