One of the most important parts of your mortgage is the mortgage rate - the rate of interest that you'll pay on the money you borrow to buy your house. Often, ads for mortgage lenders make it sound as if they offer a single mortgage rate to all lenders. If that were the truth, it would be easy to find the right mortgage - just shop around for the lender advertising the lowest interest rate and apply for a mortgage with them. Unfortunately for simplicity, calculating a mortgage rate is far more complex than that. The truth is that the mortgage rate that you're offered is influenced by many different things.

Prime Lending Rate

Mortgage lenders generally base their calculations of their mortgage rates on the prime lending rate. That's not to say that the prime lending rate is the mortgage rate that they'll offer to customers. Rather, it's the starting point of their calculations for their mortgage rates. The prime lending rate is the interest rate that most commercial banks charge their most creditworthy customers. It is adjusted up or down, usually in increments of 1/8 or ? of a percentage point. It responds to both the availability of money to loan and the demand for loans in the marketplace. Because those things tend to be the same across the board, most of the major banks will be offering the same prime lending rate.

First time borrower?

If you're a first time home buyer and your credit is good, banks and lenders will often offer mortgages at a discounted rate - one that is below the prime lending rate - in order to attract your business. First time home buyers who meet certain income guidelines may also qualify for first-time home buyer loans guaranteed by the federal government. One of the conditions of those loans is a very low interest rate, usually several points below the prime lending rate.

Your credit rating

One of the major factors that affects the mortgage rate a bank or lender will offer you is your credit rating or your credit score. Lenders use your credit score to determine whether or not they'll lend you money, and how much they'll charge you in interest for the money that you borrow. The better your credit rating, the lower the mortgage rate you'll be offered.

The type of mortgage

Different types of mortgages carry different risks for lenders. The higher the perceived risk to the lender, the more interest they'll charge you for your mortgage. Adjustable rate mortgages (ARMs) present the lowest risks to the lenders because your mortgage rate can rise if the interest rates rise. Fixed rate mortgages are riskier for lenders. They're making the gamble that interest rates won't rise above the mortgage rate that they charge you. Thus, fixed rate mortgages nearly always carry higher interest rates than adjustable rate mortgages. This can be affected by the size of the loan, and how adjustments are calculated.

The amount and length of the mortgage

It's a general but not a hard and fast rule that the larger the amount borrowed, the lower the interest rate will be. In addition, the longer the term of your mortgage, the lower the rate will be. These differences can be very slight up front, but they add up over the life of the loan. A difference of an eight of a percent can save you tens of thousands over the course of thirty years.

The amount of your down payment

In many cases, the amount that you can offer up as down payment will affect your mortgage rate. The reason is simple enough - the more you put down on your house, the more likely it is that you will not default on your mortgage. Zero-down mortgages generally carry mortgage rates that are considerably higher than the prime lending rate. Depending on the lender and the state of the economy in general when you take out a mortgage, a down payment of as little as 5% or as high as 20% may make a difference in the amount of mortgage rate that you're offered.

What about the APR?

The Annualized Percentage Rate is the total cost of the loan expressed as an annual percentage rate on the amount borrowed. The APR includes any fees that are paid in addition to the interest rate, so it may differ from the mortgage rate advertised by the lender. In the United States, lenders are required by law to disclose the cost of the loan as a standardized APR in order to make it easier for consumers to compare loans.