The effect of various parameters that limit the performance of fiber lasers are calculated for yttrium aluminum garnet (YAG) as the fiber core. The optimum core diameter, the optimum fiber length, and the corresponding output power capability are determined for a variety of optical and structural parameters of YAG, based on available data in the literature to date. It is shown that a conservative limit for YAG fiber laser is 6 kW per fiber in multimode operation, compared to 1.8 kW for a silica-based fiber laser. Furthermore, it is shown that if stimulated brillouin scattering gain, a key limiting parameter, is <10−12 m/W in YAG, as a recent publication suggests, the maximum output power is 30 kW per fiber without a significant increase in fiber core diameter. Similar gains are predicted for single-mode operation, although with much lower output power limits. A parametric study shows the relative merits of improving on various optical parameters.