In the thermal design of nuclear power reactors having parallel coolant channels, engineering hot channel factors have been established to account for small dimensional deviations from the nominal design of the reactor fuel elements resulting from manufacturing tolerances, and for departures from ideal flow conditions. A description of the various deviations from nominal likely to be encountered in a practical reactor design is presented, together with methods for estimating the magnitude of the effect of each on channel enthalpy rise, film temperature difference, and maximum heat flux. Examples are given for a geometry consisting of parallel plate type fuel elements separated by rectangular coolant channels.