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Noise-induced bit error rate (BER) is an important design constraint for RSFQ circuits. No method, however, has been reported to calculate the BER of real, multi-junction RSFQ gates. We report such a method that requires only the operating margins and the root-mean-square noise current of the device in question. Noise current is determined using the ".ac" analysis card built into SPICE circuit simulators. As usual, operating margins are also determined using SPICE. These quantities determine BER via the error function, as is appropriate for threshold dynamics. The method is general enough to be applied to any RSFQ gate and even exotic circuits such as those that use passive microstrip signal propagation. Comparison to experimental data indicates that BER in RSFQ circuits does indeed arise strictly from thermal noise.