For economical operation of the MTR, mixtures of new and used fuel must be distributed so that the required cycle time is met. An equation is derived which expresses the megawatt days possible from a new fuel charge in terms of the known life of the preceding charge, and the change in the initial fuel loading. The equation takes account of effects arising from differences in the initial U235 contents of the fuel assemblies, changes in the equilibrium concentrations of Xe135 and Sm149 during the run, and production of low cross section fission poisons. For certain conditions of common occurrence it reduces to a semi-empirical equation which has been used in the past for calculating fuel loadings. The theory can be used to derive equations for the charge life when fuels other than U235 are used.