Many internal organs in the body harbor a fluid-filled lumen. Lumen nucleation and fusion have been reported as dependent on organ-type during organogenesis. In contrast, the physics of lumen suggests that force balance between luminal pressure and cell mechanics leads to generic rules. However, this hypothesis lacks experimental evidence. Here we compare lumen dynamics for three different systems (MDCK cysts, pancreatic spheres, and epiblast model) by using quantitative cell biology, microfabrication, and theory. We report that the initial cell number determines the maximum number of lumens but does not impact the steady state, which is a final single lumen. We show that lumen dynamics is determined by luminal hydrostatic pressure. We also use MDCK cysts to manipulate cell adhesion and lumen volume to successfully reproduce the fusion dynamics of pancreatic spheres and epiblasts. Our results reveal self-organisation rules of lumens with relevance for morphogenesis and tissue engineering.