Hottel’s Rule and Penner’s Rule for estimating the total absorptivity of a homogeneous gas from a total emissivity chart or expression are generalized. The generalized rule is developed from an examination of the functional relationships between total and spectral molecular gas radiation properties. It gives a rational prescription for finding the absorptivity or emissivity at arbitrary pressure from the one-atmosphere total emissivity. Calculations of total properties from band absorption data correlations are used to find the needed map of pressure scaling exponent versus path-length-pressure product and gas temperature. Also addressed is the question of allowing for reflecting walls in the calculation of radiation heat transfer between a homogeneous gas and its enclosing walls. Both exact and high-accuracy-approximate calculations are shown to be possible using only total properties.