A time domain non-linear model is presented to predict the dynamic stiffness of rubber isolators using a FE code. The problem of simultaneously modelling elastic, viscoelastic and friction contributions is removed by additively splitting them. Viscoelastic response is modelled via a fractional derivative model, while amplitude dependency is considered through Coulomb friction elements. Nevertheless, determination of the dynamic stiffness takes long time and requires large amount of memory, as well as the estimation of the transfer function between applied excitation and resulting forces at different frequency and amplitude values. This is why a frequency domain simplified procedure is also proposed to obtain the dynamic stiffness. The technique is based on providing a generalized Maxwell model with the adequate material data set, once the dynamic strain amplitude, to which the mount is subjected, is estimated from quasi-static strain values. The methodology has proved to be efficient and easy to be applied.