Adjustable-Rate Mortgages Explained: How ARMs Actually Work
The 2008 crisis gave adjustable-rate mortgages a bad name they partly deserved and partly didn’t. The 2026 ARM is a different animal — fully amortizing, capped at every reset, and often the right choice for buyers who know they’ll move, refinance, or pay off the loan inside the fixed period. Here’s how an ARM is constructed, how the rate actually adjusts, and the scenarios where an ARM beats a 30-year fixed.
The anatomy of a modern ARM
Today’s ARMs are described with two numbers: the fixed-rate period, and the adjustment frequency after that period ends. A 5/6 ARM has a fixed rate for 5 years, then adjusts every 6 months. A 7/6 is fixed for 7 years, then 6-month resets. A 10/6 stays fixed for a full decade. All three are 30-year loans — the term length is unchanged, only the rate becomes variable after the initial period.
The fully indexed rate after the fixed period is computed from two components:
- Index — a public benchmark interest rate. Almost all post-2021 ARMs use the 30-day average SOFR (Secured Overnight Financing Rate) published by the New York Fed. Older ARMs used LIBOR; if your loan still references LIBOR, it was converted to SOFR by federal mandate in 2023.
- Margin — a fixed spread set at origination. Typical margins run 2.25%–3.00%. The margin never changes for the life of the loan.
New rate = Index + Margin, subject to the caps. If 30-day SOFR is 4.20% and your margin is 2.75%, the fully indexed rate is 6.95%.
The cap structure (the part that matters most)
Every modern conforming ARM has three caps that limit how much your rate can move. The standard structure for a 5/6 or 7/6 is written as 2/1/5:
- Initial cap (2): the most the rate can rise at the first adjustment. If your start rate is 5.5%, the first reset is capped at 7.5% no matter where SOFR is.
- Subsequent cap (1): the most the rate can change at any single adjustment after the first. Two years later, the rate can move at most 1% from the prior reset.
- Lifetime cap (5): the most the rate can ever rise above the start rate. A 5.5% start rate can never exceed 10.5%, even if the index goes to the moon.
Read the loan estimate carefully — some ARMs use a 5/2/5 or 5/1/5 structure, which is friendlier at the first reset. The cap structure is non-negotiable once the loan closes, so this is the time to ask.
Worked example: a 7/6 ARM through three resets
Loan amount $500,000. Start rate 5.5% (about 1% below the going 30-year fixed). Margin 2.75%. Caps 2/1/5. Initial monthly P&I is $2,839.
| Period | Index (SOFR) | Fully indexed | Capped rate | Monthly P&I |
|---|---|---|---|---|
| Years 1–7 (fixed) | — | — | 5.50% | $2,839 |
| Year 8, reset 1 | 4.20% | 6.95% | 6.95% (under 2-cap of 7.50%) | $3,239 |
| Year 8.5, reset 2 | 4.50% | 7.25% | 7.25% (under 1-cap) | $3,300 |
| Year 9, reset 3 | 5.00% | 7.75% | 7.75% (under 1-cap) | $3,400 |
| Worst-case lifetime | 10.00%+ | 12.75% | 10.50% (lifetime cap) | $4,124 |
At each reset, the servicer recalculates the payment to fully amortize the remaining balance over the remaining term — so a higher rate means a higher payment, but the loan still pays off on the original 30-year schedule.
Why an ARM has a lower starting rate
The 30-year fixed locks the lender into a single rate for three decades. An ARM lets the lender re-price the loan every 6 months once the fixed period ends. The borrower is paid for taking that future-rate risk, in the form of a discount on the starting rate. The 2026 spread between a 30-year fixed and a 7/6 ARM has typically run 0.50%–1.25%, depending on the slope of the yield curve.
On a $500,000 loan, a 1% rate discount over 7 years is real money. At 5.5% vs 6.5%, the ARM saves about $400/month, or $33,600 over the fixed period — before any reset risk has materialized.
When an ARM is the right choice
- You’ll move within the fixed period. The median U.S. homeowner moves every 7–8 years. If you genuinely plan to sell within 5 or 7 years, a 5/6 or 7/6 ARM is just a cheaper version of a fixed-rate loan you would have refinanced out of anyway.
- You’re confident you’ll refinance. If rates fall during the fixed period, you refinance into a new fixed loan and the ARM did its job. The risk is rates rising and you being unable to refinance economically.
- You expect a large prepayment. A vesting equity grant, an inheritance, or the sale of a previous home that will pay off the loan within the fixed period eliminates reset risk entirely.
- You’re buying a starter home. Buyers who plan to upgrade in 5–7 years often pay 1% extra on a 30-year fixed they’ll never see year 8 of.
When an ARM is the wrong choice
- Your forever home. If you have any chance of living there 10+ years, the certainty of the fixed-rate is worth the rate premium.
- Tight budget. If a 2% rate increase at first reset would break your monthly cash flow, the lower start rate isn’t worth the risk. Stress-test the worst-case payment before signing.
- The yield curve is flat. When the ARM discount over the 30-year fixed is under ~0.50%, the future uncertainty isn’t adequately compensated.
- You’re betting on a refinance you can’t guarantee. Refinances require qualifying credit, income, and home value at the future date. Job loss or a housing dip can lock you into the reset.
Reading an ARM disclosure
ARM borrowers receive a Consumer Handbook on Adjustable-Rate Mortgages and a separate ARM Disclosure at application. The latter spells out:
- The exact index and margin
- The current fully indexed rate (often higher than your start rate)
- The caps
- A worked example of the worst-case payment at each reset
- Whether the loan has a conversion option to fixed
The fully indexed rate column is the one most borrowers ignore at their peril. If your start rate is 5.5% but the fully indexed rate on the disclosure is 7.0%, the rate is going up at the first reset even if SOFR doesn’t move a single basis point.
The interest-only ARM (and why most people should avoid it)
Some lenders still offer interest-only ARMs, where the first 7 or 10 years are interest-only payments before fully amortizing. The monthly payment is dramatically lower during the IO period — but when amortization kicks in, the payment can jump 50–80% even without any rate change, because the remaining balance now has to amortize over 20 or 23 years instead of 30. These loans have a place for sophisticated borrowers with variable income (think: commission-heavy or equity-compensated). For most buyers, the payment shock at year 11 isn’t worth the early-year savings.
ARM vs fixed: the break-even framework
Here’s the simple test. Take the rate spread between the ARM and the 30-year fixed (say, 1.0%). Multiply that by your starting balance to get annual savings ($5,000 on $500,000). Multiply annual savings by the fixed period (7 years = $35,000 saved).
Then take the worst-case payment increase at first reset and multiply by 12 to get the annual cost in a bad scenario ($400/mo × 12 = $4,800/yr). How many years of bad-scenario payments would wipe out your $35,000 cushion? In this case, about 7 years.
If you’re confident you’ll be out of the loan within the fixed period plus that buffer (so 14 years here), the math clearly favors the ARM. If you might be in the loan for 20+ years, the math gets close and the fixed becomes more defensible.
Try both side by side
Run your actual scenario through our ARM vs fixed calculator — enter the start rate, the fully indexed rate, the cap structure, and how long you plan to stay. The output is the breakeven year and the dollar savings or cost in each scenario.
Still deciding between term lengths in general? See our comparison of 15-year vs 30-year fixed mortgages for the longer-term tradeoff.