Sitemap By using an interest-only mortgage calculator, you can determine how much your monthly payment would be for the loan. To use the tool, you’ll need to enter factors like the amount of the loan, the interest rate and the term for the loan into the provided spaces on the calculator. Once you enter the information, the calculator provides you with the monthly payment and an amortization schedule. You can see how different interest rates or loan terms change the monthly payment, or you can compare a regular home mortgage to the interest-only one. Some free mortgage calculators let you see how much you would pay in a year as well as monthly. You can also deduct payments from the outstanding principal to determine how lowering it affects the interest-only payment.