After a year or two of low mortgage rates, you’d think that everyone who could save money by refinancing would have done it. But no, refinancing continues to account for more than 70% of new mortgages, according to the Mortgage Bankers Association.

If you’ve thought about refinancing but put the idea on hold for a while, you’ll need to start your research from scratch, as the landscape keeps changing in unexpected ways. A while ago, for example, there was little doubt it made sense to take out a new fixed-rate loan to pay off an older adjustable loan. Now the move is open to debate.

Not long ago, nearly all the experts thought mortgage rates would climb this year. But the opposite is happening. The average 30-year fixed-rate mortgage charges a mere 4.962%, according to the survey, down from about 5.6% at the start of the year. The European debt crisis and other economic troubles have sparked a “flight to safety” that is driving investors to U.S. Treasury securities, driving up bond prices and thus reducing interest rates on bonds and mortgages.

Low rates give homeowners a chance to reduce their monthly payments. But rates have been below 6% for the past year. Why hasn’t everybody refinanced already?

Some homeowners have procrastinated. Others probably had trouble scraping together cash for application fees and closing costs. And still others may have figured they’d pay off their older loans when they sold their homes. Unable to sell in this slow market, or unable to sell for enough to pay off the old mortgage, they’ve now turned to refinancing.

Refinancing can make a lot of sense for the homeowner expecting to have the new mortgage long enough to pay off the costs of taking out a new loan. The Refinance Breakeven Calculator can help with that decision if you’re thinking of moving from one fixed-rate loan to another.

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If you have an adjustable-rate loan, the decision is trickier, because you don’t know what that loan will charge in the future. ARM resets are typically figured by adding a “margin,” such as 2.75 percentage points, to an underlying index, such as the rate on Treasury securities with one year to maturity.

Right now, many homeowners with ARMs are doing just fine, making refinancing less appealing than it might have looked a few months ago. With the one-year Treasury yielding just 0.35%, compared to 5% three years ago, an ARM with a 2.75-point margin would adjust to 3.1%. Refinancing to a fixed rate loan would mean paying nearly 2 percentage points more. For every $100,000 borrowed, payments on a 5% fixed loan would be $537, compared to $427 on an ARM at 3.1%.

Of course, you could lock into the fixed rate for the long term, while there’s no telling what an ARM will charge in the future. Use the ARM vs. Fixed-Rate Calculator to compare your current ARM with a new fixed loan. Run the numbers several times to see how small, medium or large annual increases would affect your payment.

Be sure you know just how high your ARM rate could go, based on the index and margin. Also, check to see if your ARM follows the usual practice, limiting annual increases to 2 percentage points and lifetime increases to 6 points. To assess that upside risk, you’ll have to dig up the rate your ARM charged when it was first issued.

Much can depend on the index used. Common ones are the one-, three- and five-year Treasury rate, the 11th District Cost of Funding Index (COFI) and the London Inter-Bank Offer Rate (LIBOR).

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