We present a model to describe the viscosity of liquid metals with known physical properties. The model involves two adjustable parameters to the experimental data of the viscosity, but can be related to an effective atomic diameter and the coordination number of nearest-neighbor atoms in the liquid metals. The model is applied to eleven cases of mono-atomic liquid metals. The model is capable of describing the overall experimental data of the viscosity in a wide range of temperature, predicting the mean free paths of an atom in accord to the experimental interatomic distances in the liquid metals. An extension of this model to multi-component liquid metals is also suggested.