The capacitance of a parallel-plate capacitor (or condenser) is given in SI units by

, where

is the area of each plate,

is the spacing between plates,

is the dielectric constant (relative permittivity), and

is the permittivity of free space,

farad/meter. If

is expressed in

and

in

, then

microfarads (

). Capacitance determines the quantity of positive and negative charges

that can be held on the plates by a voltage

, such that

.

In an air-gap capacitor, in which there is no dielectric layer,

.