<prompt> Human: I can find plenty of calculators on the internet that will show the effect of making an additional principal prepayment with each mortgage payment. But they all assume one fixed prepayment amount for one fixed amount of time. What I am looking for is a calculator that can handle different prepayment amounts for different time periods. For example : prepay 500 extra with each payment for 2 years, then prepay 200 extra with each payment for 5 years. The intent is to calculate how much these extra payments shorten the mortgage. I have not found anything yet that will let me do this. Can anybody point me to a calculator that will do what I want? Thanks.  <response>: The best solution I would know of is to use Excel. 