FV=PMT(1+i)((1+i)^N - 1)/i where PV = present value FV = future value PMT = payment per period i = interest rate in percent per period N = number of periods.
Even a single extra payment made each year can reduce the amount of interest and shorten the amortization, as long as the payment goes toward the principal and not the interest. Just make sure your lender processes the payment this way.