Next consider an air-core inductor, again assuming idealized geometry. The relative permeability is approximated as 1. The inductance of a helical conducting coil, as shown in the graphic, is then given by , where is the number of turns. Typical values can be in the range of microhenries (H). Considered as a solenoid, the inductor produces a magnetic field , when carrying a current . The energy of the inductor equals , which implies a magnetic-field energy density .

Combining the above results gives the well-known formula for the energy density of an electromagnetic field in a vacuum: . This is valid for electric and magnetic fields from any sources, notably for electromagnetic radiation.