Mortgage calculators are use to help a current or potential real estate owner determine how much they can afford to borrow to purchase a piece of real estate. Mortgage calculators can also be used to compare the costs or real interest rates between several different loans, determine the impact on the length of the mortgage loan of making added principal payments or bi-weekly instead of monthly payments. A mortgage calculator is an automated tool that enables the user to quickly determine the financial implications of changes in one or more variables in a mortgage financing arrangement. The major variables include loan principal balance, periodic interest rate compound interest, number of payments per year, total number of payments and the regular payment amount.