A simplified equation for determining the transverse electromagnetic mode (TEM) power penetration depth of microwaves in materials having both magnetic and dielectric response was derived. The penetration depths for a magnetite concentrate were calculated using this “full-response” equation, and a significant difference is shown compared with the penetration depth obtained using only the dielectric response (i.e., assuming no imaginary part of complex relative permeability). The temperature dependence of the power penetration depth, up to 1000°C, was determined using measured values of the real and imaginary parts of complex relative permittivity, ε_r′, ε_r″ and permeability, μ_r′, μ_r″. The accurate determination of penetration depths can help in optimizing the dimensions of a load in a microwave furnace, producing more uniform heating under microwave irradiation and avoiding thermal runaway.