Capacitance, the ability of an object or a system of objects to store an electric charge. All objects have this property to varying degrees. A familiar example of capacitance is the ability of a storm cloud to store electricity and then give it up in a bolt of lightning.

Quantitatively, the capacitance (C) of a body is defined as the ratio of the electric charge (Q) on the body to the voltage (V) of the body; that is, C = Q/V

The capacitance of a system depends upon the size, shape, and composition of the bodies in the system and their orientations with respect to one another. For example, a parallel plate capacitor—a system consisting of two identical flat, parallel metal plates separated by an insulator—has a capacitance given by the formula C = eA/d, where E is a constant that depends upon the insulator, A is the area of one of the plates, and d is the distance between the plates.

Capacitance is measured in farads (f). A parallel plate capacitor has a capacitance of 1 farad if a charge of 1 coulomb on each plate is needed to produce a voltage of 1 volt between the plates. The farad is too large a unit for practical purposes. Hence capacitance is commonly measured in millionths of a farad, or microfarads (mf).