STRATEGY PAPER

ARM vs Fixed: A Long-Term Wealth Comparison

Lower rate now. Unknown rate later.
Last reviewed March 2026 · DwellQ Research6 SOURCES

Key Findings

015/1 ARM offers 0.5–1.0% lower initial rate vs 30-year fixed
02On $400K loan, saves ~$200/mo ($12K over 5yr fixed period)
03Rate resets capped at 2%/adjustment, 5% lifetime (typical)
04Worst-case payment increase of 40–60% after fixed period
05ARM favors those who sell or refinance within the fixed period
06DwellQ’s Q+ tier models the full ARM lifecycle with caps and resets

The Rate Advantage Window

A 5/1 ARM typically offers a 0.5–1.0% lower initial rate than a 30-year fixed. On a $400K loan, a 0.75% rate difference saves approximately $200/mo ($2,400/yr) during the fixed period. Over 5 years, that’s $12,000 in reduced interest—real savings that can be invested or used to pay down principal.

Rate Reset Mechanics

After the initial fixed period, ARM rates reset based on an index (typically SOFR) plus a margin (1.5–3.0%). Resets are bounded by caps: typically 2% per adjustment period and 5% over the life of the loan. A 5.1% starting ARM rate could theoretically reach 7.1% at first reset and 10.1% at its lifetime cap—increasing monthly payments by 40–60%.

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Who ARMs Favor

ARMs favor borrowers who plan to sell or refinance before the fixed period ends. If you’re confident in a 5-year hold, the ARM’s lower initial rate reduces total interest costs. If you might stay 7–10+ years, the fixed rate eliminates payment uncertainty. The decision is ultimately a bet on your own behavior and rate direction.

Worst-Case Modeling

DwellQ’s Q+ tier models the full ARM lifecycle: initial rate, reset schedule, caps, and worst-case scenarios. This allows you to see what happens if rates rise to the cap and you can’t refinance or sell. If the worst case is manageable, the ARM’s savings during the fixed period may be worth the risk.

THE BOTTOM LINE
ARMs are a bet on your future behavior or on rate direction. DwellQ’s Q+ tier models the full ARM lifecycle with rate caps and reset schedules.

Frequently Asked Questions

Are ARMs risky?+
The risk is payment uncertainty after the fixed period. If you sell or refinance before the first reset, you capture the savings with no reset risk. If rates rise and you stay, payments can increase 40–60%.
What index do ARMs use now?+
Most new ARMs are indexed to SOFR (Secured Overnight Financing Rate), which replaced LIBOR. The margin (1.5–3.0%) added to SOFR determines your reset rate.
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RELATED ANALYSIS
When Refinancing Actually Improves Net WorthHow Interest Rates Move Your Break-Even
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METHODOLOGY
DwellQ research uses a net worth comparison framework. Both paths—buying (building equity minus all ownership costs) and renting (investing the down payment plus monthly surplus)—are modeled month-by-month over the full holding period. Assumptions are documented, sensitivity-tested, and sourced from publicly available data. This is scenario analysis, not financial advice.
SOURCES & REFERENCES
  1. Federal Reserve Bank of St. Louis. FRED: 30-Year Fixed and 5/1 ARM Mortgage Averages.[fred.stlouisfed.org]
  2. Consumer Financial Protection Bureau. Adjustable-Rate Mortgage (ARM) Handbook.[consumerfinance.gov]
  3. Freddie Mac. Primary Mortgage Market Survey, Historical Data.[freddiemac.com]
  4. Federal Reserve Board. Secured Overnight Financing Rate (SOFR) Data.[federalreserve.gov]
  5. Campbell, J.Y. and Cocco, J.F. ‘Household Risk Management and Optimal Mortgage Choice.’ QJE, 118(4), 2003.
  6. Koijen, R., Van Hemert, O., and Stanton, R. ‘Mortgage Timing.’ J. of Financial Economics, 93(2), 2009.