Adjustable-rate mortgages (ARMs) differ from fixed-rate mortgages in that the interest rate and monthly payment can change over the life of the loan. ARMs also generally have lower introductory interest rates vs. fixed-rate mortgages. Before deciding on an ARM, key factors to consider include how long you plan to own the property, and how frequently your monthly payment may change.

Why choose an adjustable-rate mortgage?
The low initial interest rates offered by ARMs make them attractive during periods when interest rates are high, or when homeowners only plan to stay in their home for a relatively short period. Similarly, homebuyers may find it easier to qualify for an ARM than a traditional loan. However, ARMs are not for everyone. If you plan to stay in your home long-term or are hesitant about having loan payments that shift from year-to-year, then you may prefer the stability of a fixed-rate mortgage.

Components of adjustable-rate mortgages
Adjustable-rate mortgages have three primary components: an index, margin, and calculated interest rate.

An ARM can also have an initial adjustment period based on a "teaser rate," which is an artificially low introductory interest rate offered by a lender to attract homebuyers. Usually, teaser rates are good for 6 months or a year, at which point the loan reverts back to the calculated interest rate. Remember, too, that most lenders will not use the teaser rate to qualify you for the loan, but instead use a 7.5% interest rate (or calculated interest rate if it is lower).

Rate caps
To protect homebuyers from dramatic rises in the interest rate, most ARMs have "caps" that govern how much the interest rate may rise between adjustment periods, as well as how much the rate may rise (or fall) over the life of the loan. For example, an ARM may be said to have a 2% periodic cap, and a 6% lifetime cap. This means that the rate can rise no more than 2% during an adjustment period, and no more than 6% over the life of the loan. The lifetime cap almost always applies to the calculated interest rate and not the introductory teaser rate.

Payment caps and negative amortization
Some ARMs also have payment caps. These differ from rate caps by placing a ceiling on how much your payment may rise during an adjustment period. While this may sound like a good thing, it can sometimes lead to real trouble.

For example, if the interest rate rises during an adjustment period, the additional interest due on the loan payment may exceed the amount allowed by the payment cap--leading to negative amortization. This means the balance due on the loan is actually growing, even though the homeowner is still making the minimum monthly payment. Many lenders limit the amount of negative amortization that may occur before the loan must be restructured, but it's always wise to speak with your lender about payment caps and how negative amortization will be handled.