First take your APR (Annual Percentage Rate) and divide it by 365 (the days in the year) to get your daily interest rate. (Note that there may be different APRs that apply to different transactions on the same bill.) Every day, your credit card issuer will multiply the daily interest rate for each transaction that hasn’t been paid off by the dollar amount of the transaction. That’s how much interest you’ll be charged for that day. This interest gets compounded, which means it’s added to what you owe. Each day, you’ll have a new daily balance, and the credit card issuer will calculate the interest on this amount. The daily interest charges are all added up to determine your monthly interest payment, which keeps compounding until you pay your bill in full.