Start by entering in the amount still owed on your loan. Then enter the payment amount that you pay on that loan each month, and finally enter in your current interest rate. You can also test adjustable rates for the future if that is the type of loan you have. For example, if you owe $324,325 on your loan, then that would be your principal. If you pay $2,784 on your mortgage per month, that would be your monthly mortgage payment, and the rate would be, for example, 6%, 7.75%, 9.25%, etc...