Your mortgage documents should have included an amortization schedule. Amortization turns a fixed interest rate and a fixed payment into a variable amount of principle and interest payment. Your amortization schedule should spell out who much goes to each for each payment you make. From there you should be able to tell how much interest you're paying. That would be the amount the bank is making off giving you a loan for your privilege of having the money now instead of saving up for some unknown number of years to have enough to buy a place.