Meaningful thresholds are essential for promoting source code metrics as an effective instrument to control the internal quality of software systems. Despite the increasing number of source code measurement tools, no publicly available tools support extraction of metric thresholds. Moreover, earlier studies suggest that in larger systems significant number of classes exceed recommended metric thresholds. Therefore, in our previous study we have introduced the notion of a relative threshold, i.e., a pair including an upper limit and a percentage of classes whose metric values should not exceed this limit. In this paper we propose RTTOOL, an open source tool for extracting relative thresholds from the measurement data of a benchmark of software systems. RTTOOL is publicly available at http://aserg.labsoft.dcc.ufmg.br/rttool.