We present a new computationally efficient and accurate model of ion concentrations in the bottomside ionosphere based on the photochemistry. There has long been a need for efficient and accurate specification of ionospheric molecular ion concentrations. Incoherent scatter radars need to specify the relative ion concentrations in order to accurately determine plasma temperatures. Full physical ionospheric models are available but too costly and cumbersome for many applications. The international reference ionosphere (IRI) model is an efficient empirical model that accurately specifies the electron density but the molecular ion concentrations are based on limited data sets. Our new ion density calculator (IDC) model uses chemical equilibrium to determine all ion concentrations except the O+ density, which cannot be derived from chemical equilibrium above ∼180 km due to the increasing importance of diffusion. The IDC model overcomes this problem by using an iterative technique to solve for the O+ density given the electron density that is provided by the radar or the IRI model and the fact that the total ion concentration must sum to the electron density. This quasi-chemical model produces very good agreement with satellite measured ion densities and significantly improves electron and ion temperatures from incoherent scatter radars. It also produces good agreement with the Field Line Interhemispheric Plasma (FLIP) physical ionosphere model, which solves the continuity, momentum, and thermal equations. Comparisons with the IRI model point out the shortcomings of the most recent version, IRI-2007 in representing molecular ion densities.