We axiomatically define a cardinal social inefficiency function, which, given alternatives and individuals' vNM preferences, assigns a number -- the social inefficiency -- to each alternative. These numbers induce not only an ordinal comparison but also a cardinal comparison between alternatives that is uniquely defined by our axioms despite no exogenously given interpersonal comparison, outside option, or disagreement point. We interpret these numbers as per-capita losses in endogenously normalized utility. We apply our social inefficiency function to a setting where interpersonal comparison is hard to justify -- object allocation without money -- leveraging techniques from computer science to prove an approximate-efficiency result for the Random Serial Dictatorship mechanism.

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