The weathering process significantly alters the mineralogical, geochemical, and physico-mechanical properties of rocks that affect the strength characteristics of rocks. In the study, three grades of weathered gneiss, i.e., fresh, slightly, and moderately weathered from Kullu, Himachal Pradesh, India, are used to correlate the rebound hammer values and porosity with chemical weathering indices. The rebound hammer is a simple and widely used equipment for assessing the strength and quality of rocks in the field. The major oxides present in all three weathering grades of gneiss were determined using X-ray fluorescence (XRF) analysis. These XRF results are further used for the calculations of chemical weathering indices. In the present study, five chemical weathering indices were evaluated for the gneiss rock of the studied area. Empirical correlations have been developed between the rebound hammer values and porosity with suitable chemical weathering indices. In addition, the petrographic analysis of thin sections of these gneisses shows the pervasive sericitization of plagioclase with progressive weathering. The extent of converted plagioclase feldspar was also reported by the increasing trend of the plagioclase index of alteration (PIA) with increasing weathering grade.