Abstract The poorly understood mechanisms of slug flow remain an obstacle for widespread application of this dense phase flow. The parameters that characterise the flow have repeatedly been observed to have a bounded range of operating conditions; however, the ability to reliably predict these boundaries has not achieved the same level of repeatability. This paper presents a model to predict the absolute maximum transport boundaries for slug velocity and the absolute minimum transport boundaries for the layer fraction as a function of the gas mass flow rate. The predicted transport boundaries are supported by measurements, following which, the model is further developed as a prediction tool, where very good agreement with measurements is achieved. As the model provides relationships between all of the key parameters of slug flow, analysis is provided demonstrating the ability of the model to be applied as a convenient design tool for slug flow pneumatic conveying systems.