When you are planning on buying a home, you will want to know how much you need to pay out each month for your mortgage. Using a house payment calculator can estimate the amount of each payment for you based on several factors. First, the amount of the loan and the amount of the down payment that you have will be entered. Then you need to enter in the interest rate as well as the repayment period. This will be required and should be entered in the number of months. The number of payments you make obviously will influence the amount of each of the payments of course. The output is an estimate but will be relatively close to the actual amount.