The hydrodynamic rainfall-runoff and urban drainage simulation model SWMM (Storm Water Management Model) is a state of the art software tool applied likewise in research and practice. In order to reduce the computational burden of long simulation runs and to use the extra power of modern multi-core computers, a parallel version of SWMM is presented herein. The challenge has been to modify the software in such minimal way that the resulting code enhancement may find its way into the commercial and non-commercial software tools that depend on SWMM for its calculation engine. A pragmatic approach to identify and enhance only the critical parts of the software in terms of run-time was chosen in order to keep the code changes as low as possible. The enhanced software was first tested for coherence against the original code and then benchmarked on four different input scenarios ranging from a very small village to a medium sized urban area. For the investigated sewer systems a speedup of six to ten times on a twelve core system was realized, thus decreasing the execution time to an acceptable level even for tedious system analysis. A parallel version of SWMM for multi-core processors is herein presented.The enhanced software was first tested for coherence and then benchmarked.Changes were kept minimal in order to encourage adoption.A speedup of six to ten times on a twelve-core system was realized.