In this paper, we address the problem of fast point-to-point channel capacity estimation in the case where the receiver undergoes unknown interference from multiple sources, whereas the channel with the transmitter is perfectly known. For this particular context, we propose a fast estimator for the capacity estimation, and compare its performance with that of the traditional methods. More precisely, we analyse the fluctuations of the traditional and proposed techniques and prove that their behaviors can be approximated by Gaussian random variables for which we derive the variances.