In this paper, we present an efficient hardware structure to estimate signal to noise ratio (SNR) for M-ary amplitude and phase modulation schemes at the receiver. The proposed hardware structure requires a counter and two comparators with a predetermined look up table (LUT). For an estimation error of less than 1 dB, it needs just about a few hundred samples. We can design the LUTs with theoretically derived values and tailor them for special purposes. We demonstrate a few simulation results of the estimation algorithm, and present hardware implementation results.