In the field of metallic materials with amorphous structures, it is vitally important to understand the glass formation and to predict glass-forming ability (GFA) in terms of constituent elements and alloy compositions. In this study, an expression has been formulated from first-principles calculations to predict the trend of GFA by hybridizing both internal energies and atomic-scale defect structures. The prediction of GFA from compositions has been verified successfully by available experimental data in the model Zr-Cu alloy system. The physical scenario revealed here has extensive implications for the design of bulk metallic glasses with superior GFA.