This paper proposes a simple series form formula of the bit error rate (BER) for M-ary QAM/OFDM signals under nonlinear fading channels. In the proposed derivation, the effect of the nonlinear distortion due to the soft envelope limiter is approximated by the additive Gaussian noise with the variance equal to the power of the clipped portion of the composite waveform. Since the BER is represented by the simple series form formula, the proposed approach frees us from the time-consuming computer simulations. Moreover, the proposed formula is applicable to arbitrary M-ary quadrature amplitude modulations (QAM). The validity of the proposed approach is confirmed by the agreement with the computer simulation results with parameters of the input back-off (IBO) and average CNR.