Singlet fission (SF) could potentially generate two triplet excitations from the absorption of a single photon, which can be applied to significantly enhance the energy conversion efficiency of solar cells. Efficiency of SF for molecular dimers depend on couplings between electronic configurations, however, available theoretical methods for quantitative evaluations of these couplings often require high computational cost and thus it is still difficult to explore geometrical dependence of SF performance for molecular dimers. Herein, we present a direct derivation of the couplings relevant to SF process on the basis of rigorously localized dimeric orbitals. We then systematically show that the SF rates can be correlated with a SF overlap factor that can be easily evaluated from monomeric molecular orbitals. Furthermore, we demonstrate the validity of the overlap factor as a quantitative descriptor of effective SF coupling based on comparative studies on slip-stacked ethylene and tetracene dimers. With the validity confirmed, we applied the descriptor to investigate a large family of polycyclic aromatic hydrocarbons to reveal dimer geometries that could potentially exhibit highly efficient SF. Our results provide a theoretical tool that could significant increase the speed and reliability of the discovery process for efficient organic SF materials.