Usability has become an essential aspect for the success of the software products. In the E-Commerce domain, the relevance of this quality attribute is even more significant. For this reason, there are methods that can be used to determine whether a software product meets the appropriate level of usability. In the category of quantitative techniques, the metrics are the most used by the specialists to establish the usability degree of a software system. However, the traditional metrics fail to cover aspects that the new emerging types of applications present. This fact leads to the postulation of new metrics that can measure the degree to which these aspects affect the usability. In this paper, the authors present a systematic review about the metrics that are reported in the literature as an assessment tool for the usability of E-Commerce websites.