To propose a method to calculate the tissue phantom ratio (TPR) using the depth dose and to compare the proposed method with two other methods.Methods: An analytical dose model from Bjärngard was used to describe the depth dose and the TPR. The parameters of the model were derived from depth dose measurements, which were then used to calculate the TPR. The calculated TPR values were compared with actual measurements as well as with TPR values predicted from two methods that also use depth dose, namely, the method proposed by BrainLAB and the conventional method that sets the quotients of the scatter phantom ratios (Sp) to 1.Results: TPR values calculated from the proposed algorithm deviated by −0.2 ± 0.1% (mean deviation) from the experimental measurements, over a range of field sizes and depths.Conclusions: The results of the proposed method were in better agreement with the experimental measurements than were results using the other two methods. Furthermore, the differences between the proposed method and the other methods are statistically significant.