The Iranian criminal justice system has traditionally relied on fixed monetary fines, a mechanism that determines penalty amounts solely based on the severity of the offense without considering the offender’s economic capacity. This study aims to critique this approach and assess the feasibility of replacing it with a “day-fine” system. The research employs a descriptive–analytical method, using library-based studies (Iranian criminal laws and regulations, judicial practice, criminological sources) and comparative analyses of successful systems such as those of Finland and Germany. Data collection tools include content analysis of legal documents, comparative field examination, and analysis of criminological and jurisprudential theories, with findings interpreted through a comparative–inferential approach. The results indicate that the fixed fine structure in Iran, by disregarding indicators such as income, assets, and living expenses, conflicts with core principles of criminal justice—including proportionality, deterrent justice, and equality before the law—and, in practice, results in unequal punitive burdens among different social classes. Comparative analysis further reveals that the day-fine model, which combines two components—“number of fine days” (aligned with offense severity) and “daily fine amount” (based on the offender’s financial capacity)—can impose an equivalent punitive burden on offenders regardless of their economic status, maintaining deterrence while preventing the reproduction of inequality. The novelty of this study lies in proposing a localized framework for implementing the day-fine system in Iran, grounded in the experiences of leading jurisdictions and an analysis of its legal and institutional capacities and challenges.