ARM Index

What Is an ARM Index?

The term ARM index refers to the benchmark interest rate to which an adjustable-rate mortgage (ARM) is tied. An adjustable-rate mortgage's interest rate consists of an index rate value plus a margin. The index underlying the adjustable-rate mortgage is variable, while the margin is constant. There are several popular indexes used for different types of adjustable-rate mortgages, such as LIBOR or the Fed Funds Rate. The interest rate on an ARM with its index is an example of a fully indexed interest rate.

Key Takeaways

  • An ARM index is a base interest rate used to compute adjustable-rate mortgage interest for some time period.
  • This index or reference rate can be the prime rate, LIBOR, or the rate on U.S. Treasury bills, among others.
  • The fully indexed ARM rate includes the index rate plus some pre-determined margin of additional interest.
  • An ARM index is different from ARM margin, which is also used to calculate the rate on an adjustable rate mortgage.

Understanding ARM Indexes

Adjustable-rate mortgages are one of the credit market's most popular variable rate products. Interest rates are fixed for the initial period of the mortgage then reset based on fluctuations in the market during the remaining lifetime of the loan. Quotes for ARMs can vary, with the first number representing the years charging a fixed rate. A 2/28 ARM would have a fixed rate for two years followed by an adjustable rate for 28 years. A 5/1 ARM could have a fixed rate for five years followed by an adjustable-rate that resets every year. This makes these mortgages ideal when borrowers believe mortgage rates will fall.

The loan is based on an indexed rate plus a margin during the variable rate period. An open variable rate increases or decreases when a change occurs with the indexed rate. If a loan has specific terms for resetting the interest rate, such as at the end of each year, then the interest rate will be adjusted to the fully indexed rate at the time of the adjustment.

The index to which an adjustable-rate mortgage is tied can make a difference over the life of the mortgage. While it is an important factor, borrowers should consider more than the index when choosing an adjustable-rate mortgage. Many other variables, such as the margin and the interest rate cap structure, are important considerations. Other factors that are important include the starting rate and the length of the loan.

While the ARM index is important, make sure you also consider other factors like the margin, the starting rate, and the length of the loan.

Types of ARM Indexes

There are several different types of ARM indexes. Each has its own characteristics that set it apart from the others. The following are some of the most popular.


The prime rate is set by the Federal Reserve and used by most financial institutions including banks and credit unions. This is the interest rate that most commercial banks charge their most creditworthy clients. It serves as the basis for other interest rates including those for mortgages and loans.

This index is typically used in the pricing of short- and medium-term loans, or for adjustments at set intervals on long-term loans. The rate is consistent on a national basis, allowing consumers to make an apples-to-apples comparison regardless of where they live. This means the prime rate is the same in California and Maine, so mortgagors can compare how competitive their adjustable-rate mortgages are in both states. The margins on the loan and whether or not the interest is set below the prime rate all become elements in comparing loan offers.


The prime rate can also be used as an index rate for establishing the APR on a credit card.


As a global index, the London Interbank Offered Rate (LIBOR) is a barometer for the global economy and is used by investors who operate internationally. This index is based on the interest rate charged among London-based banks for borrowing transactions between them. The LIBOR index is often used as an ARM index to cover intervals that can be one month, three months, six months, or one year.

As of December 31, 2021, CHF and EUR LIBOR settings, the 1 Week and 2 Months USD LIBOR settings, the Overnight/Spot Next, 1 Week, 2 Months and 12 Months GBP and JPY LIBOR settings have ceased to be published. The Overnight, 1-, 3- 6-, and 12 Months USD LIBOR settings will be published until Jun. 30, 2023, and be replaced with other benchmarks, such as the Sterling Overnight Index Average (SONIA).

Monthly Treasury Average

The Monthly Treasury Average Index (MTA) is a popular ARM index, especially for those who want to hedge against rising interest rates. This index is a moving average calculation with a lag effect. This means if interest rates are expected to rise, a mortgage tied to the MTA index may be more economical than one tied to an index without a moving average calculation like the one-month LIBOR index. But while it's a good bet when interest rates rise, it doesn't bode so well when they fall.

One-Year T-Bill

Many ARM loans use this index. This index is based on the auction results for 12-month Treasury Bills (T-Bills) held by the U.S. Treasury which are offered every week. Due to the highly fluid nature of the yields of the one-year T-bill—because of the weekly auctions—the index is much more volatile.

ARM Index vs. ARM Margin

ARM index and ARM margin represent two different elements of an adjustable rate loan. The index rate, as mentioned, is the benchmark rate that's used to determine the rate for your loan. This rate can adjust up or down over time, in accordance with changing market conditions.

Margin represents the number of percentage points that your loan rate can increase by, once the fixed-rate period ends. ARM margin is established in your loan agreement and it doesn't change once the loan closes. Where the margin is set is determined by the lender and the terms of the loan. The fully indexed rate for an ARM is equal the margin and index added together.

Example of an ARM Index

Understanding how an ARM index works is important when deciding whether an adjustable rate mortgage is appropriate, based on both its short- and long-term affordability. So, say you're planning to buy a home and you''re looking at an ARM that follows the one-year T-bill as its index.

Your loan particulars are as follows:

  • Mortgage amount: $300,000
  • Mortgage term: 30 years
  • Fixed rate period: 5 years
  • Fixed period interest rate: 3.25%
  • ARM index: 1.891%
  • ARM margin: 3.00%

The loan has an annual adjustment period with a 2% initial adjustment cap, then a 1% adjustment cap thereafter. Your estimated monthly mortgage payment for the interest and principal during the fixed-rate period would be $1,306. With a fully-indexed rate, your payment would increase to $1,549. Your total interest expense for the life of the loan would be $243,081.


What Is an ARM Index?

An ARM index is the benchmark rate that's connected to an adjustable rate mortgage. This is a variable rate that can increase or decrease over time, following the movements of current market conditions.

What Is ARM Margin and Index?

ARM margin represents the number of percentage points that an interest rate an adjustable rate mortgage can increase once the fixed-rate period ends. ARM index measures the benchmark rate that's used to calculate the fully indexed rate.

How Is ARM Calculated?

To calculate the mortgage for an adjustable rate mortgage, you would add the ARM index and the ARM margin. The sum of the ARM index and ARM margin is the fully indexed rate or the rate that is applied to your loan's monthly payments.

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  1. Consumer Financial Protection Bureau. "For an Adjustable Rate Mortgage (ARM), What Are the Index and Margin, and How Do They Work?"

  2. Consumer Financial Protection Bureau. "What is the Difference Between a Fixed-Rate and an Adjustable Rate Mortgage (ARM) Loan?"

  3. Federal Reserve. "What is the Prime Rate and Does the Federal Reserve Set the Prime Rate?"

  4. Government Finance Officers Association. "What is LIBOR?"

  5. Intercontinental Exchange, Inc. "ICE LIBOR."