The incident power density (IPD) is generally spatially averaged over an averaging area. Two quantities may be used to define the spatially averaged IPD, either using the normal component of IPD vectors crossing the surface, or magnitude of IPD vectors (norm) averaged over an area independent of the orientation. This document provides guidance for determining the appropriate definition of IPD. The guide examines theoretical, computational, and measurement data that are relevant to the validity of each method, taking into account the dependence of the IPD metric and its averaging methods on surface temperature rise in the frequency range 6 GHz to 300 GHz. This guide might be used in the development of standards for the EMF exposure limits or in the EMF compliance assessment standards.