In this paper a robust methodology for quantifying the impact of short-circuit faults on microgrids is proposed. Microgrids can operate in both islanded (grid-forming) and grid-connected (grid-following) modes, and the ownership and responsibility for the microgrid’s operation can vary significantly: from distribution system operators to third party microgrid operators. This necessitates the development of a robust short-circuit calculation (SCC) methodology that can provide accurate results for all the possible microgrid topologies, operational modes, and ownership models. Unlike previously developed SCC methods for microgrids, the methodology proposed in this paper provides highly accurate results for all possible microgrid topologies: 1) islanded; 2) grid-connected; and 3) as a part of a larger distribution grid. In addition, the proposed methodology solves short circuit faults of any complexity, with the same simplicity. The methodology is tested on a complete model of a real-life microgrid on the Case Western Reserve University campus, operating in both islanded and grid-connected modes. The computational results show the advantages of the proposed method relative to previous SCC methods for microgrids, regarding the robustness (ability to solve complex short-circuits with an arbitrary number of faulted busses and phases, that affect a microgrid of any topology), as well as the accuracy of the results.