Let's say you have $100,000 left on a mortgage. The interest rate is 3.00%. The next monthly payment is due on the 5th of February. It is now the 24h of January.
If you made an additional $5,000 principal only payment today, would the interest component of February's payment be:
A. (0.03*$100,000)/12 = $250.00
or
B. (0.03*$95,000)/12 = $237.50
I would *think* it's A because mortgage interest is paid in the arrears, and for at least one day of January, you had $100,000 on the loan.
But from experience, I think in actuality it's B, which would indicate that the interest calculation is done on payment day and is based on current numbers and not what happened during the past month.
By the way, this question is purely academic. In the long run over a 15-year mortgage, it doesn't mean shit.
Thanks for any replies.