The tutorial shows how to build an amortization schedule in Excel to detail periodic payments on an amortizing loan or mortgage. If you add an extra payment the calculator will show how many payments you saved off the original loan term and how many years that saved.This spreadsheet creates an amortization schedule for a fixed-rate loan, with optional extra payments. Adding extra principal payments in order to pay off the loan earlier than the loan contract calls for. In this tutorial we will add this feature. Download a free Home Mortgage Calculator for Excel. Analyze a fixed or variable rate mortgage with optional extra payments. Fill in the "Period" column. Fill in the column titled "Period" with your payment dates. This amortization calculator shows the schedule of paying extra principal on your mortgage over time.