Optical multipass cells are used in photochemical reactors and laser excitation of weak transitions. In these applications, estimation of the radiation dose in a volume of interest allows us to assess the performance and optimize the design of the cell.We adopt radiant fluence as the figure of merit and employ the radiative transfer equation to derive analytical expressions for average radiant fluence in a given volume of interest. These expressions involve quantities that are either obtained withMonteCarlo ray tracing methods or approximated on the grounds of geometry arguments. Furthermore,we compute the spatial distribution of fluence by applying either of the two methods to individual volume elements in a rectangular voxel grid.Ray tracing is performed with ZemaxOpticsStudio 18.9.