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How Does Fixd Work Fixed Loan Meaning How mortgage interest rates work To calculate mortgage interest, start by multiplying your monthly payment by the total number of payments you’ll make. Then, subtract the principal amount from that number to get your mortgage interest. For example, if you’re paying $1,250 dollars a month on a 15-year, $180,000 loan, you would start by multiplying $1,250 by 15 to get $225,000.