The numerical optimisation of the thorax & head dual cavity side airbag has been an integrated part of a thorax & head feasibility study for a mid size vehicle. The objective was to minimise injury parameters for the EUROSID-1 due to a side impact deformable barrier crash test. To reach this objective, the distribution of gas between the chambers of the airbag needs to be optimal. In this case, key factors that determine the gas distribution are the design parameters vent hole size, inflator output, and fabric permeability. The influence on injury results of these design parameters have been studied.