At present, the loan interest of rural credit cooperatives is usually calculated by the product method based on the actual loan days. Generally, there are two situations, namely, normal interest and overdue interest. The first one is: interest = principal * interest rate * actual loan days (which can be counted as the beginning and the end, which varies from place to place). The interest rate is generally determined by the credit cooperatives themselves according to the national listed loan interest rate, which is less than 2.3 times (allowed by the state).% is determined. ‰ is the monthly interest rate. For example, if someone borrows 1, yuan in principal on March 1, the loan interest rate is 1.5‰, and the maturity date is May 3, and it is agreed to repay the principal and interest in one lump sum, then the interest payable by the customer on the maturity date is: The principal is 1, * daily interest rate (1.5‰/3)*9 days = 3,15 yuan. If it is overdue, overdue interest can be charged. If it is agreed to collect interest on a monthly basis, the interest that is not charged on a monthly basis can be charged with penalty interest. The proportion of overdue interest and penalty interest charged is determined at the time of borrowing, which varies from place to place.