Nonlinearities in MEMS silicon resonators are caused by different effects. Depending on the resonator layout, different nonlinearities may be dominant in the resonator response. Based on experimental results, a modelling approach is proposed to arrive at a nonlinear dynamic model that potentially captures the observed behaviour. Apart from the model, consisting of a mechanical and an electrical (measurement) part, the effect of thermal noise is also estimated. With the proposed model, a quantitative match between the simulation and experimental results is established such that a good starting point is achieved for a more thorough modelling procedure.