The tutorial shows how to build an amortization schedule in Excel to detail periodic payments on an amortizing loan or mortgage. Here is the link to my spreadsheet where you can calculate a loan over x years to be paid back monthly.It will give you the amortized payment amount. I would like to be able to calculate the remaining amount of loan due after each payment on a loan with irregular payment dates and irregular amounts. Want to know the impact of "extra payments" on your mortgage? Check out this easy to use Excel mortgage calculator. 1. Download the amortization schedule from Microsoft. 2. Open the template in Excel. 3. This spreadsheet-based calculator creates an amortization schedule for a fixed-rate loan, with optional extra payments. 1) Download and open ExcelDemy's file in the Irregular Extra Payment tab.