An approximate method for estimating gamma external dose given from an arbitrary distribution of radioactive material has been developed. For the assessment of external gamma dose, the space over which radioactive material is distributed is assumed to be composed of rectangular cells. The evaluation of three-dimensional integration over the space is an extremely time-consuming task. In this study, a regular hexahedron is modeled as a volume-equivalent sphere to simplify the integration. A comparative study with a point source approximation and the Monte Carlo integration method shows that the present method gives reasonable results in a physical sense. An accident consequence assessment model has been integrated using the Gaussian puff model and an external gamma dose has been assessed for Korean meteorological data. A procedure useful for selecting the most effective protective action to a given environmental condition is also provided here.