Small-molecule organic semiconductors are used in a wide spectrum of applications, ranging from organic light emitting diodes to organic photovoltaics. However, the low carrier mobility severely limits their potential, e.g., for large area devices. A number of factors determine mobility, such as molecular packing, electronic structure, dipole moment, and polarizability. Presently, quantitative ab initio models to assess the influence of these molecule-dependent properties are lacking. Here, a multiscale model is presented, which provides an accurate prediction of experimental data over ten orders of magnitude in mobility, and allows for the decomposition of the carrier mobility into molecule-specific quantities. Molecule-specific quantitative measures are provided how two single molecule properties, the dependence of the orbital energy on conformation, and the dipole-induced polarization determine mobility for hole-transport materials. The availability of first-principles based models to compute key performance characteristics of organic semiconductors may enable in silico screening of numerous chemical compounds for the development ... mehr of highly efficient optoelectronic devices.