The electric field strength of a parallel plate capacitor is equal to the voltage across the plates divided by the distance between the plates. This relationship is written as E=V/d, where E is electric field strength, V is voltage and d is the distance between the plates.
In an ideal parallel plate capacitor, the electric field strength is constant throughout the volume between the capacitor plates. In a real capacitor, in which the plates have finite size, the electric field strength is slightly lower close to the edges of the plates. The relationship E=V/d is true away from the plate edges.