Introduction: The determination of bank interest rates in Iran, particularly in relation to investment for production, is not merely a technical issue of monetary policy but an institutional and governance oriented problem linked to the functioning of the banking system, macroeconomic stability, the structure of financing, and the requirements of a resistance economy. Most previous studies have concentrated either on global interest rate theories or on isolated aspects of Iran’s banking system, and few have offered an integrated framework that accounts for the country’s specific legal, institutional, and international constraints. To address this gap, the present study aims to identify the factors affecting the formation of bank interest rates and to clarify the conditions for achieving an optimal rate that supports productive investment within the framework of Iran’s resistance economy policies.

Methods: This developmental-applied study was conducted based on a pragmatist philosophy and a sequential exploratory mixed-methods design. In the qualitative phase, factors influencing bank interest rates and strategies for their improvement were identified through an analytical-critical review and semi-structured interviews with five experts in the fields of monetary economics, banking, and the "Resistance Economy"; these factors were then analyzed using content analysis. In the quantitative phase, a fully structured questionnaire based on the identified factors was administered to another five experts, and the importance of each factor was evaluated using the fuzzy Delphi technique. In this method, linguistic judgments were converted into triangular fuzzy numbers, defuzzified, and weighted. A value of 3 (the midpoint of the Likert scale) was set as the consensus threshold.

Finding: The content analysis revealed that interventions by supra monetary institutions in central bank independence, administrative determination of interest rate corridors, freezing of part of banks’ assets, and restrictions on international transactions have substantially weakened the possibility of discovering an optimal rate. Experts emphasised that the main challenge of productive investment is less about the shortage of financial resources and more about the quality of governance, institutional transparency, and macroeconomic stability. The fuzzy Delphi results identified six accepted determinants: firm level economic risk, repayment guarantee, producer inflation, productive sector risk, real return of the production sector, and loan maturity. Among these, producer inflation (weight = 0.197) and firm level risk (weight = 0.176) had the greatest importance. Four other variables firm capital structure, policy coordination, financial institution efficiency, and economic confidence were rejected by the expert panel. The final model demonstrates that the optimal interest rate should be determined within a gradual, transparent, risk based framework aligned with producer inflation, governance quality, and the real financing capacity of production.

Conclusions: This study provides the first integrated framework that unifies legal institutional, macroeconomic, and firm level determinants of bank interest rates in Iran within a single mixed methods model. The results confirm that interest rate setting in Iran is not merely an economic calculation but a complex governance issue requiring coordination among the three branches of government, legal reforms, mitigation of sanctions effects, and a gradual transition from administratively set rates to market discovered rates under intelligent supervision. The findings are consistent with experiences in Argentina (2023) and Russia (2022), where rate hikes were used to combat inflation and systemic risks, but they contradict the Turkish experience (2022), where rate cuts despite high inflation exacerbated the crisis. The practical implications for policymakers include redesigning the rate setting system based on producer inflation and credit risk, strengthening central bank operational independence, developing transparent credit assessment and collateral systems, expanding Islamic capital market instruments with floating rates tied to the real sector, and gradually implementing the reference rate corridor without direct interference in retail rates.