The tutorial shows how to build an amortization schedule in Excel to detail periodic payments on an amortizing loan or mortgage. Loan amortization schedule. 3.4, Enter values, Loan summary. Does anyone know how to calculate the principal and interest payments after a one time extra principal payment on a mortgage in excel? Fill in the column titled "Period" with your payment dates. Input the month and year for the first payment loan within cell A8. 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. Co-payments from the calculation of the cost of dispensing. He stated that it should be demanded of the insurance carrier to compensate for the fees for representation in the matter.