What credit card feature determines the amount of interest paid on unpaid balances?

The credit card feature that determines the amount of interest paid on unpaid balances is the Annual Percentage Rate (APR). The APR is the interest rate charged on any outstanding balance carried over from one billing cycle to the next. A higher APR results in more interest accrued on unpaid balances, while a lower APR reduces interest costs. Additionally, the way interest is calculated—whether daily or monthly—can also affect the total interest paid.