If you make two extra mortgage payments per year, you could shave several years off your repayment term and save thousands in interest. For instance, two extra annual payments on a $300,000 30-year fixed-rate mortgage at 6.75% would cut your repayment term by over 9.5 years and save more than $144,000 in interest.
2% of your repayment. Let's say you're paying on a weekly or monthly basis. Let's say monthly basis you're paying roughly $2000. If you add extra 2% under $2000, that 2% extra can save you 14 to 15 years on interest.
In summary, the payoff is calculated by adding the unpaid mortgage principal balance, adding the per-diem interest owed, and adding whatever payoff fees are charged by the mortgage servicer (typically about $100 to $150).