Some examples of determining wellbore leak crossflow rate between formations in an injection well are described. During normal operation of an injection well, multiple bottomhole pressures are determined based on multiple surface injection pressures. An Inflow Performance Relationship (IPR) for the injection well is determined based on the multiple bottomhole pressures and the multiple injection flow rates. After shut-in responsive to a subsurface leak, the shut-in injection well is modeled as a producing well having the IPR determined during normal operation of the injection well. A crossflow rate in the injection well at a location of the subsurface leak in the injection well is determined based on the IPR of the modeled producing well.