When an additional payment is applied directly to principal, it reduces the outstanding mortgage balance faster than making only the scheduled payment. A lower balance means less future interest accrues, allowing a greater portion of subsequent payments to reduce principal. Over time, this can move your expected payoff date forward. The exact change depends on your remaining balance, interest rate, loan term, and the size and timing of your extra payments. For example, consistently adding a fixed amount every month will generally have a different effect from making one large additional payment each year. To see the difference for your own mortgage, use a mortgage payoff calculator or a mortgage calculator with extra payments. When making additional payments, verify that your servicer applies the extra amount to principal rather than treating it simply as an early payment of a future installment.
