Fixed vs Adjustable Rate Mortgage

A fixed-rate mortgage locks one rate for the life of the loan; an adjustable-rate mortgage, ARM, holds a lower intro rate for 5, 7, or 10 years and then adjusts annually with the market. The ARM is a bet on your own exit date.

Real Estate & DebtLast reviewed 2026-10-07

What it is

A fixed-rate mortgage charges one interest rate for the life of the loan, and the payment never changes except for taxes and insurance in escrow. An adjustable-rate mortgage, ARM, charges a lower fixed rate for an introductory period: the 5, 7, or 10 in a 5/1 or 7/1 ARM. After the intro it adjusts once a year at an index plus a margin, with caps limiting each adjustment and the lifetime total.

The ARM's intro rate is lower than the fixed rate because the borrower carries the rate risk after the intro period. When the index rises, the payment jumps; when it falls, the payment drops. The 2006-2008 wave of ARM resets is the reference case for the wrong side of that bet, and the 2022-2023 reset wave is the recent one.

Key takeaways

A fixed rate never changes; an ARM adjusts annually after a 5, 7, or 10 year intro.
ARM adjustments are the index plus a margin, capped per adjustment and over the loan's life.
A typical cap structure is 2% at the first adjustment, 1% after, 5% lifetime.
The intro rate is lower because the borrower carries the risk after the intro ends.
The decision is the hold period: move inside the intro and the ARM wins on price.

How it works

The fixed side is one number. A $400,000 loan at 6.5% fixed for 30 years carries a payment of roughly $2,528 for 360 months, and the only variables are escrow and the payoff date. The predictability is the product: the lender absorbs the rate risk, and charges for it in the rate.

The ARM side is a formula. After the intro period, each annual adjustment is the index, typically SOFR, plus a margin set in the note, subject to caps. A common structure is 2% above the prior rate at the first adjustment, 1% at each later one, and 5% above the start rate over the loan's life. A 5/1 ARM at a 5.5% intro rate on the same $400,000 carries a payment near $2,272. If the index plus margin is 8.5% at the first reset, the cap limits the new rate to 7.5%, and the payment rises to roughly $2,800: about $528 more a month.

The caps are the floor on the damage, not a defense. A borrower who holds the ARM through three rising adjustments reaches the lifetime cap. The payment at 5% above the start rate is roughly $3,660 on this loan, about $1,380 more than the intro payment. The reset wave of 2022 and 2023, when short rates rose faster than at any time in decades, repriced exactly this arithmetic across millions of adjusting loans.

Why it matters

The ARM is a pricing tool, not a scam, and the decision is the hold period. A borrower certain to sell or refinance inside the intro period buys the lower rate and exits before the risk arrives; a borrower who might stay pays for certainty they declined. The 2006-2008 resets are the cautionary version, and the 2022-2023 wave is the recent one.

I watched a colleague named Brent (yes, that Brent) take a 5/1 ARM because the intro payment was $250 lower, then stay in the house for nine years. The resets arrived, the payment climbed $490 above the intro, and the refinance he had planned as the exit was priced at a rate higher than the one he had refinanced out of. The intro payment was the teaser; the reset was the product.

The highest-leverage thing most Americans can do is match the loan type to the honest hold period and run the reset arithmetic before signing. Compare payments with the mortgage term comparison calculator and the refinance exit in the refinance calculator, and read the schedule both loans share in the mortgage amortization entry.

Real-world examples

The 5/1 reset

A $400,000 5/1 ARM at a 5.5% intro rate carries a payment near $2,272 for five years. At the first reset the index plus margin is 8.5%, the cap limits the new rate to 7.5%, and the payment rises to roughly $2,800: about $528 more a month, $6,300 a year. The borrower's income did not change; the formula did. The cap was the only thing between the payment and the full 8.5%.

The lifetime cap

The same ARM adjusts through three rising years and reaches its lifetime cap of 5% above the start rate: 10.5%. The payment at that rate is roughly $3,660 on $400,000, about $1,380 above the intro payment. The caps are per adjustment and lifetime, and the lifetime one is the borrower's worst case, computable before signing from the note. The 2006-2008 reset wave was this arithmetic at scale, and the foreclosures were the output. The worst case is the point of the disclosure. The caps were printed in the note the whole time.

The move-inside-the-intro win

A borrower takes the same 5/1 ARM at 5.5%, saves $256 a month against the 6.5% fixed loan, and sells in year 4. The saving is $12,300 over 48 months, and the reset never arrives. The ARM won because the hold period matched the intro period, which is the entire case for the product. The same borrower staying eight years would have paid the reset three times over, and the exit date is the bet.

Common mistakes

Mistake: Taking an ARM for a long hold.

Why it bites you: The intro rate is a teaser for a borrower who stays past it, and Brent's $250 saving became a $490 increase by year nine. The product is priced for the exit, not the decade.

Mistake: Ignoring the caps in the note.

Why it bites you: The cap structure, 2/1/5 or similar, is the worst case and it is in the loan documents. A borrower who cannot afford the lifetime cap payment cannot afford the ARM.

Mistake: Assuming a refinance will be available at the reset.

Why it bites you: The 2022-2023 resets arrived with refinance rates above the teaser rates being escaped. The exit plan needs a second exit plan, because rates are not required to cooperate.

Mistake: Comparing the intro payment without the reset arithmetic.

Why it bites you: The intro payment is five or seven years of a 30-year schedule; the reset is the other 25. The comparison that matters is the intro payment against the worst-case capped payment.

Ivy's take

I spent an evening inside three ARM notes, reading the caps and the indices so you do not have to. The finding is that the ARM is honest about what it is: a lower rate for a defined period, then the market, with caps as the worst case. I ran the reset arithmetic across three rate paths, including the 2022 one. The lifetime cap payment was the number that decided every case, and it was printed in the note the whole time.

Brent took the teaser payment, stayed nine years, and refinanced at a rate worse than the one he left. The ARM was not the problem; the hold period was. Match the loan to the exit date, or pay for the fixed rate and be done.

Related jargon, defined

ARM: Adjustable-rate mortgage: fixed intro period, then annual adjustments.
Index: The market rate the ARM adjusts to, typically SOFR.
Margin: The fixed spread added to the index, set in the note.
Cap structure: The limits on each adjustment and the lifetime total, often 2/1/5.
SOFR: The secured overnight financing rate, the standard ARM index.
Rate lock: The fixed-rate guarantee; the product the ARM declines.

Frequently asked questions

What is a 5/1 ARM?+

A mortgage with a fixed rate for five years, then annual adjustments at an index plus a margin. The second number is how often the rate adjusts after the intro period.

When does an ARM make sense?+

When the honest hold period is shorter than the intro period: a borrower selling or refinancing inside five years buys the lower rate and exits before the risk. Beyond the intro, the fixed rate is the safer product.

What are ARM caps?+

Limits on how far the rate can move: typically 2% at the first adjustment, 1% at each later one, and 5% over the loan's life. The lifetime cap is the borrower's worst case, printed in the note.

Can I refinance out of an ARM before it resets?+

Yes, and it is the standard exit, but the refinance rate is not guaranteed to be lower than the teaser. The 2022-2023 resets arrived with exit rates above the teasers being escaped.

What index do ARMs use?+

Most modern ARMs adjust to SOFR, the secured overnight financing rate, plus a margin set in the loan note. Older ARMs used LIBOR before its 2023 retirement.

Disclaimer: This information is general in nature and does not constitute financial or legal advice. Always consult a qualified professional for your specific situation.

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