Non-empirical law depicting how atomic-scale friction behaves is crucial for facilitating the practical design of tribosystems. However, progress in developing a practically usable friction law has stagnated because atomic-scale friction arises from the continuous formation and rupture of interfacial chemical bonds, and such interfacial chemical reactions are difficult to measure precisely. Here, we propose a usable friction law for atomic-scale contact by using large-scale atomistic simulations to correctly measure the interfacial chemical reactions of a realistic rough surface. This friction model is effective to predict how atomic-scale friction force varies with temperature, sliding velocity, and load. As a special example, our model predicts velocity-related mountain-like temperature dependence of friction, and this prediction result is then carefully validated by comparison with ultra-high-vacuum atomic force microscopy (AFM) experiments.