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ARM Index

ARM Index

What Is an ARM Index?

The term "ARM index" alludes to the benchmark interest rate to which a adjustable-rate mortgage (ARM) is tied. An ARM's interest rate consists of an index rate value plus a margin. The index underlying the ARM is variable, while the margin is constant.

There are several famous indexes utilized for different types of ARMs, like the London Interbank Offered Rate (LIBOR) or the federal funds rate. The interest rate on an ARM with its index is an illustration of a fully indexed interest rate.

Understanding ARM Indexes

ARMs are one of the credit market's most famous variable-rate products. Interest rates are fixed for the initial period of the mortgage then reset based on fluctuations in the market during the excess lifetime of the loan. Quotes for ARMs can differ, with the first number representing the years charging a fixed rate. A 2/28 ARM would have a fixed rate for a very long time followed by an adjustable rate for a considerable length of time. A 5/1 ARM might have a fixed rate for quite some time followed by an adjustable rate that resets consistently. This makes these mortgages ideal when borrowers accept that 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 diminishes when a change happens with the indexed rate. In the event that a loan has specific terms for resetting the interest rate, for example, at the finish of every year, then the interest rate will be adjusted to the completely indexed rate at the time of the adjustment.

The index to which an ARM is tied can have an effect over the life of the mortgage. While it is an important factor, borrowers ought to think about more than the index while picking an ARM. Numerous other variables, like the margin and the interest rate cap structure, are important considerations. Other factors that are important incorporate the starting rate and the length of the loan.

While the ARM index is important, ensure that you likewise think about 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. Coming up next are probably the most famous.


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

This index is typically utilized 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, permitting consumers to make logical comparison paying little heed to where they reside. This means that the prime rate is similar in California and Maine, so mortgagors can compare how competitive their ARMs 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 contrasting loan offers.


The prime rate can likewise be utilized as an index rate for establishing the annual percentage rate (APR) on a credit card.


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

As of Dec. 31, 2021, the CHF and EUR LIBOR settings, the 1-Week and 2-Month USD LIBOR settings, and the Overnight/Spot Next, 1-Week, 2-Month, and 12-Month GBP and JPY LIBOR settings have failed to be distributed. The Overnight, 1-, 3-6-, and 12-Month USD LIBOR settings will be distributed until June 30, 2023, and be supplanted with other benchmarks, for example, the Sterling Overnight Index Average (SONIA).

Monthly Treasury Average

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

One-Year T-Bill

Many ARM loans utilize this index. This index is based on the auction results for 12-month Treasury Bills (T-Bills) held by the U.S. Treasury that are offered consistently. Due to the profoundly liquid nature of the yields of the one-year T-bill — as a result of the week after week auctions — the index is considerably more volatile.

ARM Index versus 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 utilized to determine the rate for your loan. This rate can adjust up or down after some time, as per changing market conditions.

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

Illustration of an ARM Index

Understanding how an ARM index functions is important while concluding whether an ARM is appropriate, based on both its short-and long-term affordability. Let's assume you're planning to buy a home and taking a gander at an ARM that follows the one-year T-charge as its index. Your loan particulars are as per the following:

  • Mortgage amount: $300,000
  • Mortgage term: 30 years
  • Fixed-rate period: Five 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 completely indexed rate, your payment would increase to $1,549. Your total interest expense for the life of the loan would be $243,081.


  • The completely indexed ARM rate incorporates the index rate plus some predetermined margin of additional interest.
  • An ARM index is a base interest rate used to compute adjustable-rate mortgage interest for quite a while period.
  • This index or reference rate can be the prime rate, the London Interbank Offered Rate (LIBOR), or the rate on U.S. Treasury bills, among others.
  • An ARM index is different from an ARM margin, which is additionally used to calculate the rate on an adjustable-rate mortgage.


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 the ARM margin is the completely indexed rate, or the rate that is applied to your loan's monthly payments.

What are ARM margin and ARM index?

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

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 diminish over the long run, following the movements of current market conditions.