Non-empirical law depicting how atomic-scale friction behaves is crucial to facilitate the practical design of tribosystems. However, progress in developing a practically usable friction law has stagnated because atomic-scale friction arises from the continuous forming and rupturing of interfacial chemical bonds and such interfacial chemical reactions are difficult to measure precisely. Here, we proposed 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 a velocity-related mountain-like temperature dependence of friction, and this prediction result is then carefully validated by comparing with the ultra-high vacuum atomic force microscopy experiments.