Consider two parallel steel plates with a gap between them. When a voltage is applied to one of the plates, the difference between the charges stored on the surfaces of the plates will cause an electric field to exist between them. This is a parallel-plate capacitor. Capacitance describes how the space between the two conductors affects an electric field between them, and refers to the capacity of the two plates to hold this charge. A large capacitance can hold more charge than a small one. The amount of existing charge determines the amount of current required to change the voltage on the plate.

Capacitance is measured in Farads, after Michael Faraday (1791–1867), an English physicist and chemist who did pioneering work in electricity and magnetism. One Farad is a large unit; a typical capacitor used in electronic circuitry is measured in microFarads. The capacitance changes sensed in a typical capacitive sensor are femtoFarads.