If your adjustable rate mortgage is due to adjust this year, don’t go rushing to replace it just yet. Your soon-to-adjust mortgage rate may actually go lower. It’s related to the math behind the ARM.

Conventional, adjustable-rate mortgages share a common life cycle:

1. There’s a “starter period” in which the interest rate remains fixed
2. There’s an initial adjustment period after the starter period called the “first adjustment”
3. There’s a subsequent annual adjustment until the loan’s term expires — usually at Year 30.

The starter period will vary from 1 to 10 years, but at the point of first adjustment, conventional ARMs become the same. A homeowner’s new, adjusted mortgage rate is determined by the sum of some constant, and a variable. The constant is most often 2.25% and the variable is most often the 12-month LIBOR.

As a formula, the math looks like this:

(Adjusted Mortgage Rates) = (12-Month LIBOR) + (2.250 Percent)

LIBOR is an acronym standing for London Interbank Offered Rate. It’s the rate at which banks borrow money from each other and, lately, LIBOR has been low. As a result, adjusting mortgage rates have been low, too.

Last year, 5-year ARMs were adjusting to 6 percent or higher. Today, they’re adjusting to 3.375%.

Based on the math, it may be wise to just let your ARM adjust this year. Or, depending on how long you plan to stay in your home, consider a refinance to a new ARM.  Starter rates on today’s adjustable rate mortgages are exceptionally low in San Francisco , as are the rates for fixed rate loans.

Either way, talk to your loan officer about making a plan. With mortgage rates as low as they’ve ever been in history, homeowners have some interesting options. Just don’t wait too long. LIBOR — and mortgage rates in general — are known to change quickly.

If your mortgage is set to adjust this year, the smart move may be to let it. Today’s conforming mortgages are adjusting lower than ever before — as low as 3 percent.  It may not be what you expected when you signed for your ARM several years ago.

The reason why ARMs are adjusting lower is because of how they’re made.

When conforming adjustable-rate mortgages adjust, they adjust according to a pre-determined formula. The formula is the sum of a constant and a variable.  The constant is usually 2.25 percent and the variable is a daily-changing interest rate called LIBOR.

The formula looks like this:

New Mortgage Rate = LIBOR + 2.250 percent

LIBOR is an acronym for London Interbank Offered Rate.  It’s an interest rate at which banks borrow money from each other. In Fall 2008, when Lehman Brothers fell and sparked a global banking fear, LIBOR spiked as the risk of inter-bank borrowing jumped. 

Since then, however, LIBOR is down.

Normalcy is returning to banking and the timing couldn’t be better for Los Angeles homeowners with ARMs. 15 months ago, a homeowner’s ARM may have adjusted to 6 1/2 percent.  Today, that same ARM falls to just above 3.

As a strategy play, it might make sense to let your ARM adjust. Or, because fixed rates are still near 5 percent, converting that ARM to a long-term fixed-rate product might make sense, too.  The decision is a balance between how low do you want your payment, and how long might you live in your home.  

The longer you stay, the more it might make sense to switch to fixed-rate, even though ARM rates are so low.

If you’ve got an adjusting ARM, talk to your loan officer about your choices. Once March ends and the Fed withdraws its mortgage market support, mortgage rates may rise and the fixed-rate option may be gone.