The accidental spill of volatile solvents or the release of flammable gases within equipment and buildings is likely to form fuel concentration gradients unless efficient mixing is provided. As a consequence, even small amounts of fuel can form flammable clouds, and partial volume deflagrations may occur. Nevertheless, few indications are given in international guidelines for vent sizing and only over-conservative well-mixed stoichiometric assumptions are used. In this paper, we propose a predictive methodology for the evaluation of the dynamics of partial volume deflagration, aiming at defining useful correlations for the design of vent devices, starting from the fundamental equation for the rate of pressure rise and flame propagation in closed vessel. We define a 'stratified gas deflagration index' K-G(m), where m is the filling ratio, and use it with the most common design equations for vent sizing. The approach has been validated by means of a CFD code for the simulation of stratified laminar methane-air explosion by varying both filling ratio and volume.