Macromolecular crowding profoundly shapes intracellular phase behavior, yet the absence of a universal metric for crowding strength limits our ability to compare and predict phase separation across different systems. Here, we use the well-characterized bovine serum albumin (BSA)–polyethylene glycol (PEG) system to dissect the physics of depletion-induced phase separation under crowding. By mapping full phase diagrams for BSA in the presence of PEG of varying molecular weights and polydispersity, we show that the observed behavior is quantitatively captured by Generalized Free Volume Theory (GFVT). Crucially, we identify an entropic crowd- ing parameter — the ratio of PEG volume fraction to the polymer-excluded volume fraction — that collapses the data across a range of PEG sizes onto a single master curve. This scaling captures the thermodynamics of crowding-induced LLPS independently of crowder size and covers a broad range of proteins beyond BSA, revealing that protein solubility under crowding is governed by the polymer's occupancy of accessible free volume. Our findings establish a universal control parameter for PEG-induced entropic crowding, enabling quantitative