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Home Financing Library

Adjustable Rate Mortgages (ARMs)

Adjustable rate mortgages have monthly principal and interest payments that change after a set period of time, based on an external financial index. ARMs can offer a variety of features, such as lower rates, caps on interest, and conversion options. If you plan on relocating or trading up in a few years, you may want to take advantage of an adjustable rate mortgage.

With an ARM, the interest rate changes at specified intervals (for example, every year) depending on changing market conditions; if interest rates go up, your monthly mortgage payment will go up, too. However, if rates go down, your mortgage payment will drop also.


The index of an ARM is the financial instrument that the loan is "tied" to, or adjusted to. The most common indices are the 1-Year Treasury Security, LIBOR (London Interbank Offered Rate), Prime, 6-Month Certificate of Deposit (CD) and the 11th District Cost of Funds (COFI). Each of these indices move up or down based on conditions of the financial markets.


The margin is one of the most important aspects of ARMs because it is added to the Index. This will determine the interest rate you will pay. When the margin is added to the index, it is called the “fully indexed rate”. Margins on loans may range from 1.75% to 3.50%, for example.

Interim Caps

All adjustable rate loans carry interim caps. Many ARMs have interest rate caps of six months or a year. There are loans that have interest rate caps of three years. Interest rate caps are beneficial in rising interest rate markets, but can also keep your interest rate higher than the fully indexed rate if rates are falling rapidly.

Lifetime Caps

Typically ARMs have a maximum interest rate or lifetime interest rate cap. The lifetime cap varies from company to company and loan to loan. Loans with low lifetime caps usually have higher margins, and the reverse is also true. Those loans that carry low margins often have higher lifetime caps.

Contact a loan officer to determine if an ARM is the best option for you.