In this work, a 3-dimensional (3D) ray-tracing model is proposed. The model is specifically tailored for RFID applications by taking into account some of its unique characteristics. The proposed model can give accurate power predictions and is suitable for indoor RFID systems operating with low fade margins. The model has been verified by comparison with Method of Moments (MoM) and practical measurements. It is shown to achieve a high accuracy with significantly less computational complexity than MoM. An example application of the model is presented comparing the performance of two antennas with similar beam patterns but different axial ratio. The predicted tag detection rates (63% and 50%) are in good agreement with the experimental data (65% and 55%), showing the accuracy of the model and its improvement over previous models which are not able to handle axial ratio as accurately.