Legal analysis: If there is no agreed repayment period or interest, overdue interest shall be paid. Since it is difficult for the creditor to prove the act of claiming rights from the debtor before the prosecution, if it can be proved that the borrower has not repaid after being urged, the interest can be calculated from the date of urging. If it is impossible to prove whether the creditor has urged before the prosecution, it shall be regarded as the creditor's urging, and the overdue interest shall be calculated from the date of prosecution.

If two borrowers have a dispute over the interest rate, and the agreement is unclear and cannot be proved, the interest can be calculated according to the interest rate of similar loans from banks.

Legal basis: the Supreme People's Court's Opinions on People's Courts Handling Loan Cases Article 9 For regular interest-free loans between citizens, if the lender asks the borrower to pay overdue interest, or for irregular interest-free loans that are not repaid after being urged, the lender may refer to the interest rate of similar loans from banks.

Article 676 of the Civil Code of People's Republic of China (PRC) If the borrower fails to repay the loan within the agreed time limit, it shall pay the overdue interest in accordance with the agreement or the relevant provisions of the state.