Modelling the swelling behaviour of intumescent materials is an important and challenging issue because this phenomenon influences their pyrolysis process. It must be therefore considered to improve the predictability of pyrolysis models. The objective of this work was to implement a swelling model in the pyrolysis code Gpyro in order to predict the behaviour of a sample of ethylene vinyl acetate containing aluminium trihydroxide (EVA/ATH) polymer studied by cone calorimetry. This polymer is used for making the external sheath of electrical cables. Unlike existing works in the literature, the model implemented here, and adapted to EVA/ATH, does not require information on the densities of the initial and final materials, which makes it more predictive. To characterize the swelling, experiments were carried out to measure the evolution of the sample thickness as a function of time. The mass loss and the back surface temperature of the sample were also measured. This was done under different operating conditions. These measurements are compared with the swelling predicted by the model and the results were found to be in good agreement. The model can be extended easily to other materials.