RFID systems are often used in industry to reduce costs, increase process efficiency and minimize human intervention. The challenge is to design an RFID system before it is implemented in a specific environment in the shortest possible time and at minimum cost while maintaining the accuracy of the results. In this paper, a new approach to predicting indoor UHF RFID signal coverage is presented. It is based on a graphical ray tracing method. Simulations are performed based on spatial analysis of the illumination of a 3D indoor environment created from a 2D floor plan. The results show a heat map representing the predicted RSSI radio signal levels using a color range. The approach is validated by comparison with the results of the empirical Multi-Wall model. The time complexity of the approach is presented. The proposed approach is able to generate a heat map with the accuracy of the empirical Multi-Wall model. The interior room equipment required to refine the results ought to be investigated in the future.