Virtually all credit cards use the average daily balance method including new purchases to calculate interest. (debtkid.com)
The Average Daily Balance method is one of the more common interest calculations card issuers us. (creditcardeducation.com)
Please note: Sending in a larger amount, earlier in the month, will almost always lead to a lower interest charge (provided the credit card company uses the average daily balance method for calculating interest). (ncnblog.com)