What is an Adjustable-Rate Mortgage (ARM)
An adjustable-rate mortgage (ARM) is a home loan with an interest rate that's fixed for an initial period and then adjusts periodically based on a market index — typically offering a lower initial rate than fixed-rate loans in exchange for future rate uncertainty.
An adjustable-rate mortgage (ARM) is a type of home loan where the interest rate is fixed for an initial period — typically 3, 5, 7, or 10 years — and then adjusts at regular intervals based on a benchmark market index. After the fixed period ends, your rate (and monthly payment) can go up or down depending on where interest rates are at the time of each adjustment.
ARMs are commonly described using a notation like 5/1, 7/1, or 10/6:
- The first number = years of the fixed-rate period
- The second number = how often the rate adjusts after that (1 = annually, 6 = every 6 months)
How ARM Rate Adjustments Work
When an ARM adjusts, the new rate is calculated by adding a margin (a fixed amount set by the lender, typically 2–3%) to a benchmark index (a market rate, now commonly the SOFR — Secured Overnight Financing Rate).
Example
If the SOFR index is 4.5% and your margin is 2.75%, your new rate would be 7.25%.
This adjustment can significantly change your monthly payment. A rate increase of 2% on a $350,000 mortgage translates to roughly $460 more per month.
Rate Caps: Protection Against Extreme Increases
ARMs include cap structures that limit how much the rate can change:
- Initial adjustment cap — maximum increase at the first adjustment (often 2–5%)
- Periodic cap — maximum increase at each subsequent adjustment (often 1–2%)
- Lifetime cap — maximum total increase over the life of the loan (often 5%)
A loan advertised as "2/1/5 caps" can jump 2% at first adjustment, 1% per subsequent adjustment, and 5% total above the initial rate.
ARM vs. Fixed-Rate Mortgage
| Feature | ARM | Fixed-Rate |
|---|---|---|
| Initial rate | Lower | Higher |
| Payment after fixed period | Variable | Constant |
| Risk | Rate can increase significantly | None — rate never changes |
| Best scenario | Sell or refinance before adjustment | Stay for 10+ years |
| Worst scenario | Rates spike, payment jumps | Rates fall and you miss savings (solved by refinancing) |
When an ARM Makes Sense
An ARM is most advantageous when:
- You plan to sell before the adjustment period — in a 7/1 ARM, if you sell in year 5 or 6, you benefit from the lower initial rate without ever facing an adjustment
- Rates are expected to fall — if you take an ARM while rates are high and plan to refinance when they drop, you could get a lower fixed-rate mortgage after the adjustment period begins
- The monthly savings are significant — in high-rate environments, the difference between a 5/1 ARM and a 30-year fixed can be 0.5–1.5%, which is a meaningful monthly savings
When rates are at historic lows (as they were from 2020–2021), ARMs offer minimal advantage — the fixed rate is already very low, so there's little to gain and much to risk.
Risks of Adjustable-Rate Mortgages
The 2008 housing crisis was partly fueled by ARMs (particularly subprime ones with aggressive initial teaser rates and few consumer protections). Modern ARMs have stronger consumer protections, but the core risk remains: payment shock — a sudden, large increase in monthly payment after the fixed period ends that the borrower cannot absorb.
Before choosing an ARM, calculate the worst-case scenario — divide your loan balance by what your payment would be at the lifetime cap rate. If that payment is within your budget, the risk is manageable. If it would be unaffordable, a fixed-rate mortgage provides more security.
Talk to your lender or a mortgage broker to model both ARM and fixed-rate scenarios for your specific loan amount, time horizon, and financial situation.