[header] How to calculate a house payment [title] Calculate the mortgage amount. [step] Subtract your down payment from the purchase price of the house. Add to this any closing costs you plan to finance (or roll into the mortgage). 
[substeps] Conventional mortgages typically require a 20 percent down payment. Your down payment on a federal housing authority (fha) mortgage can be as low as 3.5 percent of the purchase price.