Mortgage calculator A home is the largest purchase most of us will ever make so it's important to calculate what your payment will be and how much you can afford. The mortgage calculator will show you how much your monthly payment will be and it can also show the effect of adding extra principal payments.   Who should use a mortgage calculator? This mortgage calculator can be used by poetential home buyers as well as real estate professionals. Once a mortgage financing agreement is in place the mortgage payment calculator may still be utilized by the borrower to determine the impact that a change in interest rate may have on the mortgage payment amount.