Many banks use an "Actual/360" formula to calculate payments, while Excel's pmt function and your financial calculator use the 30/360 formula (i.e., every month earns 30 days' interest on a 360-day year).

When banks use Actual/360, it means that interest for each day is based on the nominal rate (e.g., 6.00%) divided by 360 days. When interest accrues over 365 days, this daily rate produces a bit more than 6.00% interest income for the bank.

To calculate interest, the bank uses that daily rate based on a 360-day year, times the principal balance since the previous payment, times the actual number of days elapsed since the previous payment date.

In truth, the fully amortizing payment is a little more complex, because

the algorithm factors in the 366-day leap years during the scheduled loan term. I would guess that the algorithm calculates the total actual days in the loan term, divided by the total of the nominal 360 days in each year, times the nominal interest rate. That adjusted rate may give you the same payment if you use Excel's 30/360 formula.

The only way to know for sure is to lay out a line-by-line table of the calculation in each period. Actual days between scheduled payment dates are based on the actual days in each monthly period (31 in January, 28 in February except 29 in a leap year, etc.

What really happens during the loan is that payments are not usually posted on Saturday or Sunday, so the actual loan amortization gets skewed to the following Monday or post-holiday Tuesday. Fun, huh? And, of course, the bank earns more interest on that, too.