Resistivity Index (RI) of Fontainebleau and Bentheimer sandstones was investigated at ambient and reservoir pressures down to low water saturations using the porous plate method. The measurements compared with computations directly on high-resolution three-dimensional images of the pore space using Digital Rock Physics. The numerical computations are in reasonable agreement with the experimental measurements down to water saturations as low as Sw = 10 %. The RI measurements show that both sandstones display Archie behavior at elevated pressure. However, at ambient pressure the RI for Fontainebleau sandstone deviates from Archie behavior at low water saturations. The pore-space images suggest that the deviation from Archie behavior is due to the presence of conductive percolating grain contact regions. Bentheimer sandstone for which grain contacts do not percolate, displays Archie behaviour both at ambient and elevated pressures. The present study extends previous work on RI of clean sandstones using micro-CT based analysis to high confining pressures and shows that for such sandstones the RI at elevated pressure displays Archie behavior down to low water saturations.