The ratio, σ, of the root-mean-square end-to-end distance of a polymer chain with unperturbed dimensions, < r2 >0(1/2), to that of a freely rotating chain with the same structure, < r2 >0,f(1/2), i.e. sqrt(< r2 >0/< r2 >0,f), in the limit of infinite chain length. The steric factor reflects the effect of hindrance to free rotation.