Soft errors due to cosmic rays cause reliability problems during lifetime operation of digital systems, which increase exponentially with Moore's law. The first step in developing efficient soft error tolerant schemes is to analyze the effect of soft errors at the system level. In this work, we develop a systematic approach for soft error rate estimation. Experiments on benchmark circuits and comparison of the results with random fault injection (previous work) show that our proposed method is on average 95% accurate while 4-5 orders of magnitude faster.