Due to the rapid increase in electricity consumption and the need to increase the share of renewable sources of energy, there is a tendency to install distributed generations (DGs) at the level of distribution systems. The allocation of DGs has to be optimally determined to reduce system power losses. In this paper, an analytical formula (AF) is developed for loss reduction due to inserting multiple DG units. This analytical formula achieves the optimal placement of multiple DGs. The derived analytical formula considers the effects of the DGs during the placement of each unit. Based on this analytical formula, an iterative method is proposed for the sizing of the multiple DGs. In addition, mathematical proofs are provided to estimate approximately the effects of the load level, along with branch resistance, substation transformer tapping and DG power factor on the DG allocation. Applications on IEEE 33 and 69-bus distribution test systems are implemented for their original and simultaneous reconfiguration with DGs. The results of AF are compared with that given in the literature. Accordingly, the superiority of achieving the proved optimal solution is validated along with reducing the computational steps.