A multi-sided platform can only succeed if a critical mass of users can join. This is a necessary, but not sufficient, condition for a platform’s success. However, there is a limited understanding of the factors that contribute to reaching such critical mass. In this study we identify ways to determine the market potential of a platform and to reach critical mass. We particularly enrich past studies by exploring how the openness of a platform influences market potential. We examine openness at three levels – provider, technology, and user level – and ask the question: to what extent can opening (or closing) each level increase or decrease a platform’s market potential? The provider level recognizes the strategic involvement of key stakeholders that provide a platform. The technology level is concerned with the interoperability of a platform across different technologies. The user level relates to what extent a platform discriminates different segments of the customer base. On the basis of analytical modeling and theoretical analysis, we formulate four propositions concerning the effects of openness on platforms’ market potential. We illustrate the strength of propositions through a confirmatory case study, which is informed by five theoretically sampled cases. The cases illustrate cogently the effects of opening different levels of a multi-sided platform. In conclusion, we propose a decision model that can assist decision making concerning the opening of a platform to catalyze its growth.