I have always heard you say that if you make one extra mortgage payment per year, you can knock off seven years on the back end of a loan. If we put $1,800 (two payments) toward principal only next month how many payments can we knock off? Does the math work the same? We are in year 13 of a 30-year mortgage. Our loan balance is about $75,000. You’re asking for a simple answer to a more complicated question. In general, if during the entire term of a 30-year loan you make an extra payment each year to the lender, you will shorten the length of the loan term by about seven years.