You want to buy a house for 300,000 and pay 20% down. How much is your down payment? Calculate the principal for the home loan after the down payment. The bank tells you the total interest for the loan will be $232200 over 30 years. Calculate the total amount owed to the bank including principal and interest. Calculate the monthly payment for a 30 year mortgage. Calculate the total monthly cost including another 300 per month for taxes and 150 per month for insurance.