A credit card interest calculator can estimate the interest and total cost of repaying a balance using details such as your balance, APR, monthly payment and any new spending. Enter the figures from your latest statement, remembering that your card provider may calculate interest daily and apply it monthly, so the result is only an illustration. Check whether the calculator includes fees, promotional rates and minimum-payment changes, as these can affect the estimate. For the most accurate figure, review your card agreement or ask your provider.