Structural geometry, electronic band gaps, density of states, optical and mechanical properties of double perovskite halides ${\mathrm{Cs}}_2{\mathrm{InBiX}}_6$ (X = F, Cl, Br, I) are investigated using the density functional theory. These compounds possess genuine perovskite stoichiometry, evaluated using various geometry-based indices like tolerance factor, octahedral factor, and formation energy. The fundamental electronic band gaps are direct and valued in the range 0.80–2.79 eV. These compounds have narrow band gaps (except ${\mathrm{Cs}}_2{\mathrm{InBiF}}_6$) due to strong orbital coupling of the cations. The valence band maximum and conduction band minimum are confirmed to be essentially of In 5s and Bi 6p characters, respectively. The splitting of Bi 6p bands due to strong spin-orbit coupling causes reduction in the band gaps. These compounds have large dispersion in their bands and very low carrier effective masses. The substitution of halogen atoms has great influence on the optical properties. The mechanical properties reveal that ${\mathrm{Cs}}_2{\mathrm{InBiX}}_6$ (X = F, Cl, Br, I) satisfy the stability criteria in cubic structures.