A procedure is described for calculating interior daylight illumination using an inexpensive programmable hand calculator. The proposed procedure calculates illumination at any point within a room utilizing sky luminance distribution functions that are consistent with the CIE (Commission Internationale de lEclairage) Overcast and Clear Sky functions. This procedure separates the light reaching the point being considered into three components, these being (a) light directly from the sky, (b) light after being reflected from external, and (c) internal surfaces. Finally, two examples are presented in order to demonstrate the proposed procedure and indicate the speed with which the calculations may be performed.