Most credit card agreements assess monthly interest, if interest is assessable for the month, based on use of the average daily balance, which requires a more detailed computation than the simple computation example provided in your post.
When calculating interest based on the average daily balance, you divide your APR by the number of days in the year, then multiply that daily periodic rate by the average daily balance. The average daily balance divides the sum of your closing balance each day by the number of days in the billing cycle.
Then multiply that average daily interest calculation by the number by the number of days in your monthly billing cycle to arrive at the monthly interest.
