The High Energy Laser End-to-End Operational Simulation (HELEEOS) off-axis scattering algorithm is designed to predict the irradiance that will be detected at a given off-axis location due to atmospheric scattering of a high-energy laser. The HELEEOS system models the propagation of the laser through the atmosphere, accounting for such effects as turbulence, thermal blooming, and atmospheric absorption. The HELEEOS off-axis scattering algorithm uses the scattering phase functions of the Mie scattering models to predict the amount of radiation that will be scattered toward a particular observation location from each point along the beam path, and the total irradiance that will be received at that location. Algorithm outputs were compared with data from a laser test conducted at Wright-Patterson Air Force Base using 1.53-1.59 m communications lasers operating at 0.6 W of power. The off-axis irradiance was measured using an Indium Gallium Arsenide (InGaAs) camera.