The theoretically determined slope efficiency and threshold pump power for continuous wave (CW) operation of a Tm-doped silica fiber laser are presented. The associated rate equations are solved using standard techniques and, in conjunction with the published and our measured spectroscopic parameters as input, the model was used to examine the fiber laser output for a variety of fiber and pump configurations. After good agreement was achieved between the model calculations and the published experimental measurements, the model was used to examine the relative performance of the fiber laser when the pump wavelength was varied over the 3F4, 3H5, and 3H4 absorption bands of Tm3+. The calculated maximum slope efficiencies were determined to be approx. 40, approx. 57, and approx. 84%, respectively, for each of the 3F4, 3H5, and 3H4 absorption band pump schemes and the threshold pump power over the range of pump schemes was determined to vary by only 28%. The model was further used to analyze the fiber laser output when the fiber length, Tm3+ concentration and 3H4 energy level lifetime were varied and the consequences on the operation of the fiber laser with these variations are discussed.