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Abstract A pure theoretical investigation of convective rain formation processes and formation efficiency (FE) is performed using a kinematic one‐dimensional time‐dependent model with warm rain microphysics. FE is defined as the ratio of total cloud‐to‐rainwater conversion to total condensation. FE is a component of precipitation efficiency, which is an important but poorly understood parameter in idealized climate models. This model represents a cloud by a cylindrical thermal bubble rising at constant velocity. The model focuses on the interaction between auto‐conversion, collection, and lateral mixing about which no theory has been proposed. Taking the auto‐conversion threshold into account, a criterion for rain formation and a semianalytical approximate solution of FE are found. The auto‐conversion threshold limits the temporal and spatial extent of the “vigorous rain formation region” where most of the rain is produced. The collection and auto‐conversion compete with lateral mixing to determine the strength of rain formation within this region. The FE is predicted to be most sensitive to auto‐conversion threshold, fractional entrainment rate, and initial bubble water vapor density.