Originally Posted by ziggy29

I think if you don't pay in full by the due date, interest is retroactively charged on the average balance in the preceding period. (Don't recall the exact calculations.)

So if you had an average balance of $4000 in the preceding period, you would be charged 2% (1/12 of the 24% annual rate) per month, or in this example, $80 in interest charges. At least it goes something along those lines.