Lending a personal loan with interest is half of the principal. According to relevant data, the interest rate stipulated by the state cannot exceed 3 times of the national interest rate. At present, the interest rate of time deposits in China is about 1% to 2%, the interest rate of loans is about 4% to 5%, and the highest interest rate of personal loans lent to others is 12% to 15% of the principal. And the lender shall not include the interest in the principal to calculate compound interest, otherwise it will not be protected by law.