Interest for overdue repayment of private loans shall be calculated according to the agreement between the parties in the contract. If there is no agreement between the parties, it may be calculated according to the relevant provisions of the state. If the borrower fails to repay the loan within the agreed period, , a certain overdue interest needs to be paid. Legal basis: Article 676 of the Civil Code: If the borrower fails to repay the loan within the agreed time limit, he shall pay overdue interest in accordance with the agreement or relevant national regulations. Article 678 The borrower may apply to the lender for an extension before the expiration of the repayment period; if the lender agrees, the extension can be made. Article 667 A loan contract is a contract in which the borrower borrows money from the lender and returns the loan and pays interest when due.