Dare to Dive into ARMs? The Surprising Truth About Adjustable Rate Mortgages

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There’s no denying that navigating the world of mortgages can be tricky. One of the most misunderstood types is the Adjustable Rate Mortgage (ARM). However, understanding how ARMs work can potentially save you thousands over the life of your loan.

Demystifying Adjustable Rate Mortgages

An Adjustable Rate Mortgage, commonly referred to as an ARM, is a type of mortgage where the interest rate is not fixed but changes over time. The fluctuation in interest rate is tied to an index, such as the U.S. Prime Rate or the London Interbank Offered Rate (LIBOR), and a predetermined margin set by the lender.

How to know which index the ARM is tied to?

This information should be explicitly stated in your loan agreement. In the sections detailing the interest rates, adjustments, and calculation method for those adjustments, the specific index should be mentioned.

If it’s not clear, don’t hesitate to ask your lender to clarify. After all, understanding how your interest rate will adjust is key to being prepared for future changes in your mortgage payments.

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An Adjustable Rate Mortgage (ARM) can be tied to various indexes worldwide. Which index your ARM is tied to is typically determined by the country where the mortgage is issued. Here are a few commonly used ones:

  1. U.S. Prime Rate: This is a commonly used index in the United States. The Wall Street Journal determines the U.S. Prime Rate by calculating the rate set by the majority of the nation’s top 25 banks.
  2. London Interbank Offered Rate (LIBOR): Historically, this has been a popular index, representing the short-term interest rate at which banks lend to each other in the London market.
  3. Secured Overnight Financing Rate (SOFR): This rate is based on transactions in the Treasury repurchase market, where banks and investors borrow or loan Treasuries overnight.
  4. Treasury Index (T-Bill): Some ARMs are tied to the rate of Treasury securities, which are backed by the U.S. government. They can be indexed to the one-year Treasury Bill or other periods.
  5. Cost of Funds Index (COFI): Used particularly for ARMs with monthly rate adjustments, this reflects the interest cost incurred by financial institutions in the 11th Federal Home Loan Bank District.
  6. Constant Maturity Treasury (CMT): This index is the weekly or monthly average yield on U.S. treasury securities adjusted to a constant maturity.
  7. Certificate of Deposit Indexes (CODI, COFI): These rates are tied to the average interest rate that banks are offering on certificate of deposits.
  8. Eleventh District Cost of Funds Index (COFI): This is a weighted average of the cost of borrowing for savings institutions in the 11th district of the Federal Home Loan Bank (Arizona, California, Nevada).
  9. Maturity Treasury Average Index (MTA or MAT): This is a 12 month moving average of the monthly average yields of U.S. Treasury securities adjusted to a constant maturity of one year.
  10. Euro Interbank Offered Rate (EURIBOR): This is a daily reference rate based on the averaged interest rates at which Eurozone banks offer to lend unsecured funds to other banks in the euro wholesale money market.
  11. Tokyo Interbank Offered Rate (TIBOR): Used in Japan, TIBOR is the rate at which banks are willing to lend to each other. There are two types: the Japanese Yen TIBOR derived from a group of domestic banks, and the Euroyen TIBOR derived from a group of foreign banks.
  12. Reserve Bank of India (RBI) Repo Rate: In India, some variable-rate loans are linked to the RBI’s repo rate, the rate at which the central bank of India lends money to commercial banks.
  13. Singapore Interbank Offered Rate (SIBOR): In Singapore, the SIBOR is used as an index for some adjustable-rate loans. It’s the rate at which banks in Singapore are willing to lend to each other.
  14. Cost of Funds Index (COFI): Used predominantly in Australia, the COFI is a monthly weighted average of the interest rates paid on deposits by banking institutions.
  15. Canadian Prime Rate: In Canada, the prime rate is often used as an index for adjustable-rate loans. It is a benchmark set and used by major banking institutions.
  16. European Central Bank (ECB) Main Refinancing Operations Rate: In the European Union, this rate set by the ECB is often used for adjustable-rate loans.

The Structure of ARMs

ARMs have a structure typically expressed as two numbers, for example, a 5/1 ARM. The first number indicates the initial fixed-rate period, and the second number shows how often the rate adjusts after the initial period. So, a 5/1 ARM means the rate is fixed for the first five years, then adjusts annually thereafter.

The Initial Fixed-Rate Period

The first number in the structure denotes the length of time (in years) during which the interest rate is fixed.

For example, if you’re considering a 5/1 ARM, the “5” indicates that the interest rate will remain the same for the first five years of the loan. During this initial fixed-rate period, your monthly mortgage payments remain the same, just like a traditional fixed-rate mortgage. This period offers a level of predictability and stability for the borrower.

The Adjustment Period

The second number in the structure tells you how often the interest rate will adjust after the initial fixed-rate period ends. This is typically represented in years.

Continuing with our 5/1 ARM example, the “1” means that after the initial five years, the interest rate will adjust once every year. This adjustment is based on a specific index (such as the U.S. Prime Rate or the London Interbank Offered Rate) plus a preset margin determined by the lender.

Other Key ARM Terms

To truly understand ARMs, it’s essential to familiarize yourself with some additional terms:

Rate Caps

Rate caps limit how much your interest rate can increase. There are typically two types of rate caps:

  1. Periodic Rate Cap: This cap limits how much your rate can change in a single adjustment period.
  2. Lifetime Rate Cap: This cap limits how much your rate can increase over the life of the loan.

For example, if your loan has a 2/6 cap, the rate can rise or fall by up to 2% per adjustment period, but can’t exceed 6% over the life of the loan.

Adjustment Index

The adjustment index is a benchmark interest rate used to adjust the ARM’s interest rate. Lenders add a certain number of percentage points (the margin) to the index rate to determine the new rate.


The margin is a fixed percentage point amount added to the index that remains constant over the life of the loan.

How ARMs Impact You

How an ARM impacts you primarily depends on changes in interest rates. Let’s delve deeper into what this might look like.

When Rates Go Up

The most obvious risk of an ARM is that your interest rate, and therefore your monthly mortgage payment, can increase. If the index to which your ARM is tied rises, your interest rate will rise too. It’s essential to understand this potential risk and ensure you can afford potential increases in your monthly payment.

Let’s imagine you’ve entered the adjustable period of your ARM, and due to economic factors, the index your rate is tied to rises. This means your interest rate will increase at the next adjustment date. But what does this mean for you?

Higher Monthly Payments

With a higher interest rate, the amount of interest you owe each month increases. This leads to a higher monthly mortgage payment. This is a significant risk of ARMs – they can lead to ‘payment shock’ if there’s a steep increase in your monthly payment that you weren’t expecting or budgeting for.

Here’s a simple illustration: if your initial interest rate was 3% on a $200,000 mortgage, your monthly payment might be around $843. However, if the rate rises to 5%, your monthly payment could increase to around $1074, a significant jump.

More Interest Over the Life of the Loan

If your rate goes up and stays up, you’ll end up paying more in interest over the life of the loan compared to if you’d taken out a fixed-rate mortgage at a lower rate.

When Rates Go Down

On the flip side, if interest rates go down, so does your mortgage payment. This is one of the main reasons borrowers opt for ARMs. If you believe interest rates will fall, or you plan to sell your house or refinance before the fixed-rate period ends, an ARM can save you money.

Adjustable Rate Mortgages (ARMs) come with their fair share of risks, but they also hold potential benefits. If interest rates fall, your ARM could lead to lower monthly payments. 

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While we’ve previously explored the risks when rates rise, falling rates can be advantageous for the borrower.

Lower Monthly Payments

If the reference index decreases, the interest rate on your ARM will likely decrease as well when it comes time for an adjustment. A lower interest rate translates into a lower amount of interest due each month, which means lower monthly mortgage payments.

For example, imagine you have a $200,000 ARM, and the interest rate decreases from 5% to 3%. Your monthly payment might fall from approximately $1074 to around $843. That’s an extra $231 in your pocket each month!

Lower Total Interest Paid

A lower interest rate doesn’t just reduce your monthly payments — it can also reduce the total amount of interest you’ll pay over the life of the loan. If the interest rate decrease is significant or lasts for a long period, you could save thousands of dollars in interest.

Weighing the Pros and Cons

Like any financial decision, ARMs come with both benefits and drawbacks. Let’s consider both sides of the coin.

Advantages of ARMs

  1. Lower Initial Interest Rate: ARMs often start with a lower interest rate than fixed-rate mortgages, potentially saving you money early on.
  2. Lower Initial Monthly Payments: Due to the lower initial interest rate, your monthly payments at the beginning of an ARM are typically lower.
  3. Potential for Lower Payments in the Future: If interest rates drop, your monthly payment could decrease.

Some borrowers opt for ARMs because they believe that interest rates will fall in the future. Predicting future interest rates can be tricky and is generally best left to professionals. However, if you have a reason to expect rates to decrease, an ARM might save you money.

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Disadvantages of ARMs

  1. Uncertainty: While ARMs can go down, they can also go up. If rates rise, so will your mortgage payment.
  2. Complexity: ARMs can be more complex and harder to understand than fixed-rate mortgages.
  3. Potential for Payment Shock: If interest rates increase dramatically, your monthly payment could rise significantly, leading to ‘payment shock.’

Tips for Navigating ARMs

Understanding ARMs is crucial, but so is knowing how to navigate them. Here are some practical tips.

Read the Fine Print (Always!)

Ensure you understand the terms of your ARM, including the initial rate, adjustment periods, rate caps, and what index your rate is tied to.

  • Initial Interest Rate: This is the starting interest rate of your ARM. It’s typically lower than fixed-rate mortgages, which can be enticing. However, remember that this rate will adjust after a certain period.
  • Adjustment Period: This defines how often your interest rate will adjust. For instance, in a 5/1 ARM, your rate will adjust every year after the first five years.
  • Index and Margin: Your new interest rate during the adjustment period is determined by adding a “margin” to an “index” rate. The index rate is a broader measure of interest rates, often based on rates of U.S. Treasury securities. The margin, added to the index rate, is a set number that remains constant over the life of the loan.
  • Rate Caps: These are limits on how much your interest rate can increase or decrease. There are two types: a) Periodic adjustment cap, limiting how much your rate can change at each adjustment; b) Lifetime cap, limiting how much your rate can change over the life of the loan. These caps provide some protection against dramatic rate increases.
  • Payment Caps: Some ARMs also include a cap on how much your monthly payment can increase at each adjustment. However, be aware that if the interest rate raises more than the payment cap allows, the difference will be added to the loan balance, leading to “negative amortization,” where your loan balance grows over time instead of decreases.
  • Prepayment Penalties: Some loans charge you a penalty for paying the loan off early, which could affect you if you plan to refinance or sell the home before the end of the mortgage term.
  • Conversion Clause: Some ARMs have a feature that allows you to convert the loan from an adjustable rate to a fixed rate. The fine print will state if your ARM has this feature, when you can use it, and what the cost is.

Plan for Possible Rate Increases

Consider the worst-case scenario. Can you afford your monthly payments if interest rates rise to the maximum amount?

  • Budget for the Worst-Case Scenario: When considering an ARM, calculate the maximum possible monthly payment you could end up paying if the interest rate hits the cap. This will help you understand if you can afford the loan in the worst-case scenario. If this amount exceeds your budget, reconsider your mortgage options.
  • Build an Emergency Fund: Having a sizeable emergency fund can act as a financial cushion. If interest rates rise significantly and increase your monthly payments, an emergency fund can help you cover these unexpected costs without causing financial strain.
  • Pay Down Principal: If possible, consider making additional payments to reduce the principal of your loan. By decreasing your principal, you can reduce the amount of interest you pay over the life of the loan, making potential rate increases more manageable.
  • Consider Refinancing: Keep an eye on market conditions. If it looks like interest rates will rise significantly, you may want to consider refinancing to a fixed-rate mortgage. This can protect you from future rate increases, providing more predictability in your monthly payments.
  • Invest Wisely: If you opted for an ARM because the initial lower payments made sense for your financial situation, consider investing the difference between the ARM payment and what a fixed-rate mortgage payment would have been. If done wisely, this could result in returns that help offset future rate increases. But remember, investments can be risky and it’s important to seek professional financial advice.

Regularly Review Your Mortgage

Keep an eye on current interest rates and consider refinancing if it looks like rates will rise significantly.

ARMs can also be a good choice if you plan to sell your house or refinance your mortgage before the end of the initial fixed-rate period. In this case, you benefit from the lower initial interest rate of the ARM without facing the risk of potential rate increases in the future.

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In Conclusion

ARMs are not for everyone, but for some, they can offer significant savings. The key is understanding how they work and carefully considering both the potential benefits and risks. By making informed decisions, you can leverage ARMs to your advantage, potentially saving money and making your dream home a reality.

Keep in mind that the mortgage market is complex, and what works best for you will depend on your unique situation. Consulting with a trusted financial advisor or mortgage professional can provide personalized advice tailored to your specific needs and goals. With the right knowledge and advice, you can navigate the world of Adjustable Rate Mortgages with confidence.

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