Abstract: During injection of carbon dioxide (CO2) into deep saline aquifers, the available pore volume of the aquifer may be used inefficiently, thereby decreasing the effective capacity of the repository for CO2 storage. Storage efficiency is the fraction of the available pore space that is utilized for CO2 storage, or, in other words, it is the ratio between the volume of stored CO2 and the maximum available pore volume. In this note, we derive and present simple analytical expressions for estimating CO2 storage efficiency under the scenario of a constant-rate injection of CO2 into a confined, homogeneous, isotropic, saline aquifer. The expressions for storage efficiency are derived from models developed previously by other researchers describing the shape of the CO2-brine interface. The storage efficiency of CO2 is found to depend on three dimensionless groups, namely: (1) the residual saturation of brine after displacement by CO2; (2) the ratio of CO2 mobility to brine mobility; (3) a dimensionless group (which we call a “gravity factor”) that quantifies the importance of CO2 buoyancy relative to CO2 injection rate. In the particular case of negligible residual brine saturation and negligible buoyancy effects, the storage efficiency is approximately equal to the ratio of the CO2 viscosity to the brine viscosity. Storage efficiency decreases as the gravity factor increases, because the buoyancy of the CO2 causes it to occupy a thin layer at the top of the confined formation, while leaving the lower part of the aquifer under-utilized. Estimates of storage efficiency from our simple analytical expressions are in reasonable agreement with values calculated from simulations performed with more complicated multi-phase-flow simulation software. Therefore, we suggest that the analytical expressions presented herein could be used as a simple and rapid tool to screen the technical or economic feasibility of a proposed CO2 injection scenario. [Copyright &y& Elsevier]