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.13, Payment number, Payment date, Beginning balance, Scheduled payment, Extra payment, Total No. Due Date, Due Payment, Extra Payment (Recurring), Extra Payment (Irregular), Interest Paid, Principal Paid, Balance. 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. 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.