Auto transformer rectifier units (ATRUs) are commonly used in aircraft applications such as electric actuation for harmonic mitigation due to their high reliability and relative low cost. However, those components and the magnetic filter components associated to it are the major contributors to the overall size and weight of the system. Optimization of the magnetic components is essential in order to minimize weight and size, which are major market drivers in aerospace industry today. This requires knowledge of the harmonic content of the current. This can be obtained by simulation, but the process is slow. In order to enable fast and efficient design space exploration of optimal solutions, an algebraic calculation process is proposed in this paper for multi-pulse ATRUs (e.g. 12-pulse and 18-pulse rectifiers), starting from existing solution proposed for 6 pulse rectifier in the literature. The method consists of mapping AC and DC sides of a balanced 6N-Pulse system into the sum of equivalent 6-Pulse systems. This problem can then be solved with available methods, and then the AC and DC currents are transformed back into the 6N-Pulse problem. The method is fast and accurate, and allows calculating both magnitudes and phase angles of AC and DC currents, thus enabling reconstruction of the current waveform.