This article analyses pay determination as a process of commensuration as well as a process in which commensuration can fail. The analysis is based on an empirical study of two collective firms in Germany and the United Kingdom and their attempts to self-determine fair pay. Due to the formal equality of members and their democratic decision-making processes, these cases are a specifically interesting context for studying the determination of pay. Through the analysis of a failed attempt at finding a formula for fair pay, as well as a fragile compromise formula, a contribution is made to the literature on commensuration and the construction of compromises. The article also extends this literature by explaining the obstacles to the creation of a compromise that would go beyond the need for a common interest. Callon and Muniesa’s work on calculation is used to clarify the steps that are necessary to move from questions of worth to the assessment of worth and its expression in measures. To introduce the question of legitimacy in evaluation processes, Callon and Muniesa’s framework is supplemented with Boltanski and Thévenot’s work on critical capacities.