Your monthly principal and interest charges are determined by the rate and the amount of the loan. The rate and loan amount, in turn, are affected by several factors. The rate depends on your credit score, discount points you might pay, and whether the down payment is less than 20% of the home’s value. The loan amount depends on the size of the down payment and the home’s price. There is also the matter of private mortgage insurance, which is levied on borrowers who make a down payment of less than 20%.