In this paper, we develop low-complexity methods to compute the soft symbol and variance estimations for QAM constellations with applications to iterative MMSE-SIC receivers. In particular, we propose a bit flipping based estimation scheme. For an N-QAM constellation with Gray mapping, we derive efficient approaches for both soft symbol and variance estimations with the complexities of O(log N) and O(log2 N), respectively. We also present an improved approximation approach which completely removes the multiplications at a cost of a slight performance degradation. We later extend the proposed methods to the non-squared QAM.