Fixed vs. Adjustable-Rate Mortgage: How ARMs Work and Who They Suit

A fixed-rate mortgage keeps the same interest rate for the entire loan. An adjustable-rate mortgage (ARM) starts with a fixed rate for a few years, then changes periodically based on a market index. ARMs often start with a lower rate than a comparable fixed loan, which makes them tempting. The question is whether that early discount is worth the uncertainty later. This guide explains how ARMs work and how to judge the risk.

How a fixed-rate mortgage works

With a fixed-rate loan, your principal and interest payment is the same in month one and month 360. Your total housing payment can still change if property taxes or homeowners insurance rise, since those are often paid through escrow, but the loan portion does not.

The appeal is predictability. You know exactly what the loan will cost, and if rates fall significantly you may be able to refinance. If rates rise, you are protected.

How an adjustable-rate mortgage works

An ARM is described with two numbers, such as 5/6 or 7/1:

• The first number is the number of years the initial rate is fixed.
• The second number is how often the rate adjusts after that. A "6" usually means every six months; a "1" means once a year.

After the fixed period, the new rate is calculated as:

New rate = index + margin, subject to caps

• Index: a market benchmark that moves with interest rates. Many newer U.S. ARMs use SOFR (the Secured Overnight Financing Rate), often a 30-day average.
• Margin: a fixed number of percentage points set in your loan documents, such as 2.75. It does not change.

ARM caps: the guardrails

Caps limit how much the rate can change. They are often written as three numbers, for example 2/1/5:

1. Initial adjustment cap (2): the most the rate can rise at the first adjustment.
2. Periodic cap (1): the most it can rise at each later adjustment.
3. Lifetime cap (5): the most it can ever rise above the starting rate.

Cap structures vary by loan, so read yours carefully. Some ARMs also have a floor that limits how far the rate can fall.

Worked example (illustrative numbers only)

These rates are hypothetical and are not current offers.

You are borrowing $400,000 for 30 years and compare:

• 30-year fixed at 6.75%: principal and interest about $2,594 per month, forever.
• 5/6 ARM starting at 6.00% with 2/1/5 caps: about $2,398 per month for the first five years.

The ARM saves about $196 per month during the fixed period, roughly $11,800 over five years. After five years of payments, the ARM balance would be about $372,000.

Now stress-test the reset. Suppose rates have risen and the first adjustment hits the initial cap:

• At 8.00% on the remaining balance over 25 years: about $2,873 per month, roughly $475 more than before the reset and about $280 more than the fixed loan.
• If rates keep rising and the ARM reaches its 11.00% lifetime cap: about $3,648 per month.

If rates fall instead, the ARM payment could drop. The point of the exercise is not to predict which will happen, but to confirm you could survive the bad case.

Who an ARM can suit

• Buyers who expect to move or refinance before the fixed period ends. If you are confident you will sell in five years, a 7-year ARM may cover your entire time in the home.
• Borrowers with rising, reliable income who can absorb higher payments later.
• People who will prepay aggressively during the fixed period and want the lower rate while they do.

Who should be cautious

• Anyone who could not afford the payment at the lifetime cap.
• Buyers stretching to qualify. An ARM used only to squeeze into a bigger home is a red flag.
• People counting on refinancing later. Refinancing depends on rates, home values, income, and credit at that future date, none of which are guaranteed.

Questions to ask a lender about an ARM

1. What index does this loan use, and what is the margin?
2. What are the initial, periodic, and lifetime caps?
3. What would my payment be at the first adjustment if the index stayed where it is today?
4. What is the maximum possible payment, and when could it first occur?
5. Is there a prepayment penalty?
6. Is there a rate floor?

U.S. lenders must give you disclosures about how an ARM works. The Consumer Financial Protection Bureau's consumer handbook on adjustable-rate mortgages is a useful plain-language companion.

FAQ

Is an ARM cheaper than a fixed-rate mortgage?

At the start, an ARM rate is often lower. Over the full life of the loan, it depends entirely on where rates go after the fixed period. If rates rise, an ARM can end up costing more than a fixed loan would have.

Can I refinance an ARM into a fixed-rate loan?

Usually, if you qualify at the time. You will pay closing costs, and the fixed rate available then may be higher or lower than today's. Plan your budget as if refinancing might not be possible.

What does "5/6 ARM" mean?

The rate is fixed for the first five years, then adjusts every six months for the remaining term, within the limits set by the loan's caps.

What does 5/6 mean?

Fixed for 5 years, then adjusts every 6 months.

What do 2/1/5 caps mean?

Up to +2% at the first reset, ±1% at each later reset, never more than 5% above the start rate.

Can I refinance out of an ARM?

Usually yes, but it depends on rates, home value and credit at that time, so don't count on it.

Compare the payments

Use the Rateboard mortgage payment calculator to compare the fixed-rate payment with the ARM's starting payment, then rerun it at the capped rates to see your worst-case payment. The mortgage rates page shows survey averages for fixed and adjustable loans with their as-of dates.

Rateboard guides are educational and are not financial advice. ARM terms vary; rely on your lender's disclosures.