Adjustable Rate Mortgage (ARM)

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A Variable or adjustable loan is a loan whose interest rate, and accordingly monthly payments, fluctuate over the period of the loan. With this type of mortgage, periodic adjustments based on changes in a defined index are made to the interest rate. The index for your particular loan is established at the time of application.

Well known indices include :

  • 1. Treasury Security Indexes -- Yields on United States Treasury Securities adjusted to constant maturities. When using Treasury Securities, the ARM's adjustment period is usually the same as the security's constant maturity.
  • 2. Treasury Bills -- Commonly called T-bills they come in denominations of 3 months, 6 months and 1 year. Depending on which three of these security index schedules you choose, the interest rate on your Adjustable Rate Mortgage (ARM) will adjust once every six months, once each year, or once every three years.
  • 3. London Inter Bank Offering Rates (LIBOR) -- Interest rates at which international banks lend and borrow funds in the London interbank market.
  • 4. Certificate of Deposit Indexes -- Average rates that you get when you invest in a 1- , 3- or 6-month CD.
  • 5. 11th District Cost of Funds Index (COFI) -- This index reflects the weighted-average interest rate paid by 11th Federal Home Loan Bank District savings institutions for savings accounts and other sources of funds. ARMs based on this index can adjust every month, every six months, or every year.
  • 6. Prime Rate -- An interest rate offered to banks best customers.

Historical and current values for some ARM's indexes are available. In the H15 Federal Reserve statistical release and in business newspapers.

New interest rate = index + margin
The margin is fixed percentage points added to the index to compute the interest rate. The result will then be rounded to the nearest one-eighth of a percent.
Example:

  • The index is 5.3% and the margin is 2.5%,
  • then the new interest rate = 5.3% + 2.5% = 7.8%.
  • The nearest to 0.8% is 0.75% = 6/8%.
  • The result will be 7.75%.

The margins remain fixed for the term of the loan and are not impacted by the financial markets and movement of interest rates. Lenders use a variety of margins depending upon the loan program and adjustment periods. Most ARMs have an interest rate caps to protect you from enormous increases in monthly payments. A lifetime cap limits the interest rate increase over the life of the loan. A periodic or adjustment cap limits how much your interest rate can rise at one time.

Examples:

1. The initial interest rate is 4.5%, the index is 7%, and the margin is 3%, then the new interest rate = 7% + 3% = 10%. If the lifetime cap is 5% then the actual new interest rate will be 4.5% + 5% = 9.5%.

2. The initial interest rate is 6%, the index is 5%, and the margin is 3%, then the new interest rate = 5% + 3% = 8%. If the periodic cap is 1% then the actual new interest rate will be 6% + 1% = 7%.

Your mortgage disclosure will tell you the exact index, to be used, whether the weekly or monthly value applies, the lead time for your index, the margin, and any caps.

Some types of ARMs offer payment caps, which limit the amount the monthly payment can increase. If a loan has payment cap but has no periodic interest rate cap, then the loan may become negatively amortized: if the interest rate increases and the monthly mortgage payment does not increase sufficiently then the payment does not cover the interest payment, so the loan balance increases. However, you always have the option to pay the minimum monthly payment, or the fully amortized amount due.

With most ARMs, the interest rate can adjust once a year, every three years, or every five years. The interest rate on negatively amortized loans can adjust monthly. 1-year ARM means a loan with an adjustment period of one year.

Some types of ARMs offer an initial lower interest rate than the fully indexed rate (index plus margin) for the first six month, or the first year. It is also known as teaser rate.

All ARMs are available with 30-year terms and some with 15-year terms.
Adjustable rate mortgages generally have a lower initial interest rate than fixed rate loans.