Predicting the performance of a proposed lighting design is an integral part of the design process, allowing the designer to examine and compare alternatives, refine a promising idea, see whether applicable recommendations and codes will be met, evaluate energy conservation and lighting control opportunities, invoke standardized procedures to predict glare and visibility, and perhaps generate a rendering of how a space might appear. The purpose of this Lighting Science document is to provide the theoretical basis for lighting calculations, to describe how this theory is approximated and used, and to describe how it is embodied in most lighting analysis software. This can provide, from a user's perspective, an understanding of the power and limitations of calculations—however performed—and thus make clear their use in the lighting design process. In addition, this document explains IES standard calculation procedures, including lumen method coefficients of utilization and glare assessment.