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DC short-circuit current calculation is necessary to design and protect modular multilevel converter-based (MMC-based) multi-terminal DC (MTDC) grids. In this paper, a general and analytical short-circuit current calculation method is proposed for bipolar MTDC grids. For pole-to-pole (P2P) faults, the superposed fault equivalent network of the MTDC grid is first developed in the frequency-domain. Then, a decoupling method based on the network inductance ratio is proposed, which simplifies the high-order network into the superposition of several independent two-order RLC circuits. As a result, the short-circuit current can be expressed as the sum of the currents of these RLC circuits. For P2G faults, the phase-domain DC line impedance considering the coupling effects is determined based on its modal-domain parameters, so that the P2G fault current can be calculated using the method of the P2P fault. The proposed methods effectiveness is verified through simulations and experiments, and the calculation error is classified and analyzed. Compared with the existing methods, the proposed method is more accessible, more accurate, and available for hand-solution.