Find an online mortgage calculator with amortization and extra payment calculations, such as the calculators from the Bankrate.com and Mortgage-X websites. You can find links to these in the Resources section.
Enter your current mortgage data and calculate the principal and interest payment. You need the original principal amount, the interest rate, the mortgage start or first payment date and the mortgage term in years. Most mortgages have 30-year or 360-month terms.
Look at the amortization table for your mortgage and write down the date of the last payment and the total interest paid over the term of the mortgage. With a 6 percent mortgage, you will pay more interest than principal over the life of the loan.
Enter additional principal payments and calculate a new amortization schedule. The mortgage calculator will allow you to set up additional payments, monthly or annual, or a one-time additional principal payment. Note the effect on the total interest paid and the payoff date for the mortgage.
Work with the calculator to see the effects of different levels of additional principal payments to your mortgage.