The penetration depth is calculated according to equation (2), which shows how it depends on the dielectric properties of the material. The penetration depth is used to denote the depth at which the power density has decreased to 37 % of its initial value at the surface.

Materials with higher loss factor ε r ’’ (imaginary part of the complex permittivity) show faster microwave energy absorption. The power density will decrease exponentially from the surface to the core region.