An adjustable rate mortgage is one in which the rate changes based on the market interest rates. The rate will adjust on a specific schedule, say once a year, after an initial fixed period. Fixed periods range from six months to five years. Some may have even longer fixed periods.

The risk in an ARM comes from having a payment that can change significantly. When you have a fixed rate mortgage, you know that your payment will be the same now, ten years and twenty years later. The payment doesn't change because the interest rate is fixed.

When you choose an adjustable rate mortgage, you accept the risk of a rising payment in return for a lower initial interest rate. This rate is usually much lower than the market rate for a 30-year fixed rate mortgage. The more risk you accept, the lower your initial interest rate. The more adjustments the loan will go through, the more risk. The traditional thinking is that even after a loan adjustment, the rates will be lower than those offered to new borrowers for 30-year fixed mortgages. However, it does happen where this gap closes, especially in periods of rising interest rates.

The best time to get an ARM is when interest rates are on the decline. Despite the risk, an ARM can be beneficial to certain borrowers. While most advisors will tell you that a fixed-mortgage is the way to go in every situation, there are times when you should consider an adjustable rate.

1. The borrower needs extra cash for a while.

A lower initial fixed rate gives you more money in your pocket early in your loan term. For example, a one-year ARM with a 30-year term and a rate which adjusts once a year on the anniversary of the loan date comes with zero points and an initial rate of 5.625%. Let's compare that to a 30-year fixed rate mortgage with no points and a fixed rate of 7.625%.

If you take out a $240,000 mortgage, the 30-year fixed rate payment would be $1,698.70 each month. The one-year ARM would have a monthly payment of $1,381.58. That's a difference of $317 a month.

You could use that extra $317 to pay off your credit cards, make improvements to the home or save for retirement. But you want to make sure that you will maintain a lifestyle that can afford for your payment to increase. You don't want to find that you cannot afford a higher mortgage payment when the rate adjusts upwards.

2. Buy more home.

Because of the lower initial interest rate, you can qualify for a larger mortgage amount and a more expensive home. Many homebuyers secure a one-year ARM with the purpose of refinancing them later. The low rate allows a more costly home, but a low mortgage payment. But remember that refinancing comes with closing costs. Do the math to see if you are really saving any money.

3. It all depends on the future.

If you plan to move or upgrade in the next few years, an ARM is a wise decision. You can benefit from a lower rate mortgage and simply sell the home and buy another before the rate adjusts. For example, if you plan to move in three years, why not go in for a five-year adjustable mortgage. You get a lower rate that won't adjust while you own the home, as long as you sell during the initial rate period.

Make sure that the loan comes with no prepayment penalties. Make sure that you do some math. If interest rates go up drastically in those three years, when you buy a new home, you will be facing the higher interest rates. This could mean that you are unable to really upgrade to a larger or more expensive home.

Adjustable-rate mortgages are basically all about weighing the risk. You are getting a lower interest rate and payment for taking the risk of having to pay a lot more in the future. Some homeowners are experiencing this right now as foreclosures are on the rise. Many homeowners failed to calculate how much their mortgages could adjust to. Some have seen large increases that they are unable to afford. Do all of the math and always prepare for the worst case scenario when considering an adjustable rate mortgage.