Abstract This study aims to determine the impact of an M&A on the performance of the consolidating bank in relation to its competitors in both the intermediation and profitability approaches, in addition to proposing an M&A ranking metric for a more accurate performance analysis. Although some studies assess the performance of banks in M&As, the explanation of their impact on institutions in the market can be better explored by proposing a financial-operational metric, different from valuations by the stock market, complemented with the point of view of organizational strategies that appear in the literature as essential for the continuity of the resourcefulness of the topic. Assessing the consequences of bank M&As and understanding their intentions provides better management of both institutions and the country's socioeconomic aspects. It also provides guidelines and new tools for performance evaluation for academics, managers, and government officials on M&A processes. The research method is quantitative-qualitative. In the quantitative approach, the data envelopment analysis (DEA) technique with a second stage of multiple linear regression was used. The database was formed by information from the banks' financial statements from 2000 to 2018 (representing the CAMELS), supplemented with macroeconomic and market structure information. In the qualitative approach, the impact of these regressed variables was determined in the quantitative stage for four case studies, which are the Big4 banks that represent approximately 87% of the Brazilian financial market. Although some studies deal with the topic of the relationship between efficiency and M&As, this study shows that there is no clear and direct relationship between M&As and the performance (efficiency) of banks in the market. The results suggest that banks carry out M&As for reasons other than the search for efficiency, leaving efficiency as a consequence of the operation. This article contributes to the proposal of a new metric for measuring the efficiency of M&As, whose function is to establish a ranking and relativize the analysis of banks' performance in relation to the market as a whole, allowing the visualization of the relative effect vis-à-vis competitors.