How Is Credit Card Interest Calculated?

There is no certain duration during which the interest on your credit card is being calculated. It can be on a daily basis, on a monthly basis and on yearly also. But the most common among them is the daily basis. But this interest is not  charged on  day one when you have taken credit; the period starts at a certain date that is  stipulated by the company at the time of giving you the credit card.
     
You can better understand the phenomenon with an example. If you use your credit card on the tenth of a month and the date when they start charging you is the thirty-first, then they will not charge you for the twenty-one days and if you pay the money in that same period, there is no need to pay interest as there is no interest charged on this amount.

Interest on credit cards is calculated how many ways

It is calculated based on how much money you still owe multiplied the APR usually...