SignificanceSelf-organization through the phase separation of biomolecular condensates is ubiquitous in living cells. The macroscopic physical properties of these condensates play an essential role in their organization and function—what general principles relate these macroscopic properties to the underlying microscopic features of biomolecules? By using universal ratios of thermodynamic quantities in the vicinity of a critical point, condensate physical properties can be inferred from a small number of thermodynamic parameters, which we demonstrate in the case of surface tension. Importantly, we confirm via simulation that the range of validity of the critical region is large enough to cover the physiologically relevant range in living cells. This simplification allows us to identify key features of biomolecules that influence surface tension.