Adjustable Rate Mortgage. The consumers guide to adjustable rates.

An adjustable rate mortgage (ARM) is a mortgage loan where the interest rate on the note is periodically adjusted based on a variety of indices. Among the most common indices are the rates on 1-year constant-maturity Treasury (CMT) securities, the Cost of Funds Index (COFI), and the London Interbank Offered Rate (LIBOR). A few lenders use their own cost of funds as an index, rather than using other indices. This is done to ensure a steady margin for the lender, whose own cost of funding will usually be related to the index. Consequently, payments made by the borrower may change over time with the changing interest rate (alternatively, the term of the loan may change). This is not to be confused with the graduated payment mortgage, which offers changing payment amounts but a fixed interest rate. Other forms of mortgage loan include the interest only mortgage, the fixed rate mortgage, the negative amortization mortgage, and the balloon payment mortgage. Adjustable rates transfer part of the interest rate risk from the lender to the borrower. They can be used where unpredictable interest rates make fixed rate loans difficult to obtain. The borrower benefits if the interest rate falls and loses out if interest rates rise.

Adjustable rate mortgages are characterized by their index and limitations on charges (caps). In many countries, adjustable rate mortgages are the norm, and in such places, may simply be referred to as mortgages.


Rates for some common indexes used for Adjustable Rate Mortgages (1996-2006)

All adjustable rate mortgages have an adjusting interest rate tied to an index.

In Western Europe, the index may be the ECB Refi rate (where the mortgage is called a tracker mortgage), TIBOR or Euro Interbank Offered Rate (EURIBOR).

Six common indices in the United States are:

  • 11th District Cost of Funds Index (COFI)
  • London Interbank Offered Rate (LIBOR)
  • 12-month Treasury Average Index (MTA)
  • Constant Maturity Treasury (CMT)
  • National Average Contract Mortgage Rate
  • Bank Bill Swap Rate (BBSW)

In some countries, banks may publish a prime lending rate which is used as the index. The index may be applied in one of three ways: directly, on a rate plus margin basis, or based on index movement.

A directly applied index means that the interest rate changes exactly with the index. In other words, the interest rate on the note exactly equals the index. Of the above indices, only the contract rate index is applied directly.

To apply an index on a rate plus margin basis means that the interest rate will equal the underlying index plus a margin. The margin is specified in the note and remains fixed over the life of the loan. For example, a mortgage interest rate may be specified in the note as being LIBOR plus 2%, 2% being the margin and LIBOR being the index.

The final way to apply an index is on a movement basis. In this scheme, the mortgage is originated at an agreed upon rate, then adjusted based on the movement of the index.Unlike direct or index plus margin, the initial rate is not explicitly tied to any index; the adjustments are tied to an index.

Basic features of ARMs

The most important basic features of ARMs are:

  1. Initial interest rate. This is the beginning interest rate on an ARM.
  2. The adjustment period. This is the length of time that the interest rate or loan period on an ARM is scheduled to remain unchanged. The rate is reset at the end of this period, and the monthly loan payment is recalculated.
  3. The index rate. Most lenders tie ARM interest rates changes to changes in an index rate. Lenders base ARM rates on a variety of indices, the most common being rates on one-, three-, or five-year Treasury securities. Another common index is the national or regional average cost of funds to savings and loan associations.
  4. The margin. This is the percentage points that lenders add to the index rate to determine the ARM's interest rate.
  5. Interest rate caps. These are the limits on how much the interest rate or the monthly payment can be changed at the end of each adjustment period or over the life of the loan.
  6. Initial discounts. These are interest rate concessions, often used as promotional aids, offered the first year or more of a loan. They reduce the interest rate below the prevailing rate (the index plus the margin).
  7. Negative amortization. This means the mortgage balance is increasing. This occurs whenever the monthly mortgage payments are not large enough to pay all the interest due on the mortgage. This may be caused by the payment cap contained in the ARM when are high enough that the principal plus interest payment is greater than the payment cap.
  8. Conversion. The agreement with the lender may have a clause that allows the buyer to convert the ARM to a fixed-rate mortgage at designated times.
  9. Prepayment. Some agreements may require the buyer to pay special fees or penalties if the ARM is paid off early. Prepayment terms are sometimes negotiable.

It should be obvious that the choice of a home mortgage loan is complicated and time consuming. As a help to the buyer, the Federal Reserve Board and the Federal Home Loan Bank Board have prepared a mortgage checklist.

Limitations on charges (caps)

Any mortgage where payments made by the borrower may increase over time brings with it the risk of financial hardship to the borrower. To limit this risk, limitations on charges—known as caps in the industry—are a common feature of adjustable rate mortgages. Caps typically apply to three characteristics of the mortgage:

  • frequency of the interest rate change
  • periodic change in interest rate
  • total change in interest rate over the life of the loan, sometimes called life cap

For example, a given ARM might have the following types of caps:

Interest rate adjustment caps:

  • interest adjustments made every 6 months, typically 1% per adjustment, 2% total per year
  • interest adjustments made only once a year, typically 2% maximum
  • interest rate may adjust no more than 1% in a year

Mortgage payment adjustment caps:

  • maximum mortgage payment adjustments, usually 7.5% annually on pay-option/negative amortization loans

Life of loan interest rate adjustment caps:

  • total interest rate adjustment limited to 5% or 6% for the life of the loan.

Caps on the periodic change in interest rate may be broken up into one limit on the first periodic change and a separate limit on subsequent periodic change, for example 5% on the initial adjustment and 2% on subsequent adjustments.

Although uncommon, a cap may limit the maximum monthly payment in absolute terms (for example, $1000 a month), rather than in relative terms.

ARMs that allow negative amortization will typically have payment adjustments that occur less frequently than the interest rate adjustment. For example, the interest rate may be adjusted every month, but the payment amount only once every 12 months.

Cap structure is sometimes expressed as initial adjustment cap / subsequent adjustment cap / life cap, for example 2/2/5 for a loan with a 2% cap on the initial adjustment, a 2% cap on subsequent adjustments, and a 5% cap on total interest rate adjustments. When only two values are given, this indicates that the initial change cap and periodic cap are the same. For example, a 2/2/5 cap structure may sometimes be written simply 2/5.