Detailed description:

1. Banks usually calculate interest based on overdraft amount and overdue days.

2. If 400 yuan is in arrears for more than seven days, the bank can calculate the interest as follows:

-First, determine an overdraft interest rate. The interest rate can be fixed or floating according to the market interest rate.

-Then, calculate the overdraft amount multiplied by the overdraft interest rate to get the daily interest.

-Finally, multiply the daily interest by the number of days overdue to get the total interest.

3. The specific interest depends on the bank's policies and contracts. Please consult the specific credit card issuing bank for specific information.

Summary:

Credit cards that owe 400 yuan more than seven days will be charged a certain interest. The calculation method of specific interest varies from bank to bank. Please check the relevant policies with the credit card issuing bank.

Extended data:

1. Different banks may have different methods for calculating overdue interest. Please confirm with the card issuer for details.

2. Overdue repayment may lead to damage to credit records and may affect future credit applications.

3. In order to avoid overdue interest and credit record damage, it is suggested to repay the loan on time or negotiate with the bank in advance to postpone it.