Permittivity is the relationship between the electric flux density and the electric field strength of a capacitor with a certain dielectric material, i.e.,

where Q is the charge, V is the potential difference, d is the thickness of the dielectric in the direction of flux, and a is the cross-section area perpendicular to the direction of flux. C is the capacitance of the dielectric. The permittivity of a material is defined relative to the permittivity of a vacuum (or air as a good approximation) that is the permittivity of free space.

ε_{0} has the value 8.85 × 10^{−12} Fm^{−1}.

The relative permittivity, ε_{r}, is the ratio of the capacitance of a capacitor having a certain material as dielectric to that of a capacitor with vacuum as dielectric. This statement is embodied in the relationship

The absolute permittivity of a dielectric is therefore

Values of relative permittivity of common insulating materials are in the range 2-10 (e.g., bakelite, 4.5-5.5; glass, 5-10) and of air is 1.0006.