The appropriate sizing of the strutting sections of a tunnel depends on the adequacy of the parameters initially used to perform the calculation. A very common practice in underground engineering is to make an estimation of the strutting through the use of geomechanical classifications. Nowadays, the most widely used classifications are Bieniawski’s RMR Quality Index and The Barton et al. Q Quality Index.The research work carried out aims to provide a correlation between the two quality indexes, with the intention of supplying a tool to aid in both the design phase and the daily control of work execution. The formulas were obtained from the on-site observation of rocky masses of low and very low quality, so the application range extends to that kind of soils.Key words: Geomechanics, tunnelling, underground construction, strutting, quality index, RMR-index, Q-index.