Lenders choose different methods for calculating accrued interest because financial history, product design, legal frameworks, and operational costs all shape what is practical and market-accepted. Variation in the day-count convention and accrual rules changes the measured interest for a given period, which affects pricing, risk allocation, and comparability across products. These conventions are not arbitrary; they reflect trade-offs among accuracy, simplicity, and standardization that institutions and regulators have historically accepted.
Market practice and product design
Different markets and instruments favor different conventions. Short-term money markets often use ACT/360 because it slightly increases quoted interest relative to a 365-day base, which matters for interbank and repo trades where quoting and liquidity are paramount. Longer-term bonds and some consumer loans use 30/360 conventions because quarterly and semiannual coupon schedules are easier to administer and fit accounting systems. John C. Hull at the University of Toronto discusses how standard market conventions reduce operational friction and make pricing models consistent across institutions, especially for derivatives and fixed-income instruments.
Legal, regulatory, and cultural factors
Legal contracts, tax rules, and prudential regulation can mandate or encourage particular accrual methods. Regulators and supervisors influence lending practices because differences in accrual affect reported yields and therefore capital calculations. Frederic S. Mishkin at Columbia University explains that market conventions play a role in how policy rates transmit to lending rates, so regional differences—for example between Anglo-American markets and continental European markets—persist because of historical legal traditions and consumer protection norms. Cultural expectations about fairness and transparency also shape what conventions are acceptable in retail lending.
Consequences for borrowers, lenders, and markets
Different accrual rules create real-world consequences. Borrowers may face small but meaningful differences in effective cost between lenders that appear to quote similar nominal rates. Lenders gain or lose relative revenue depending on chosen conventions and the timing of cash flows. At market scale, inconsistent conventions complicate comparison, create potential arbitrage, and raise operational costs for institutions servicing cross-border portfolios. In lower-income or seasonal economies lenders may adopt daily accrual or flexible schedules to match borrower cash flows, introducing a territorial and human dimension to the choice. Understanding these forces explains why accrual rules persist: they balance precision, administrative ease, regulatory compliance, and the cultural expectations of the markets they serve.