To calculate the new capacitance of a capacitor with a dielectric inserted between its plates, treat the system as two capacitors: one with the dielectric and one without. When a dielectric does not touch the plates, it can be considered as three capacitors by introducing thin metal plates at the boundaries. The capacitance can be calculated using the series formula for capacitors. The formula for capacitance with a dielectric is KAE0 / Kd-x(K-1), where K is the dielectric constant, A is the area, and d is the distance between plates. When dealing with multiple dielectrics, divide the system into individual capacitors and calculate their total capacitance in series.