How to model a US ARM mortgage before you borrow
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By Editorial team
A $360,000 conventional 5/1 ARM at 5.90% saves $200/month versus a 30-year fixed at 6.75% in the first five years. Over the modeled 30-year life, scheduled interest is about $71,878 lower on the ARM. But that saving depends on where the index sits after year five. This is for buyers who want to see both the upside and the adjustment risk in one calculator view before signing.
The scenario modeled
Both scenarios use a $360,000 conventional loan with a May 2026 start date, no taxes or insurance. The 5/1 ARM includes index, margin, and cap assumptions so post-year-five adjustments appear in the schedule alongside the fixed loan for direct comparison.
| Input | 30-year fixed | 5/1 ARM |
|---|---|---|
| Loan amount | $360,000 | $360,000 |
| Start rate | 6.75% | 5.90% |
| Initial fixed period | Full 30 years | 5 years |
| Post-fix adjustments | N/A | Annual with caps |
The findings
The ARM's $200/month early advantage accumulates to about $12,000 in savings by the end of year five. Whether that advantage holds depends on where the SOFR index sits in 2031. At the modeled adjustment path, the ARM ends up about $71,878 cheaper over 30 years. If the index rises 1.5% above that path, the advantage shrinks considerably. Use the second tab to stress-test that scenario before deciding.
About the modeled path: The lifetime-interest numbers assume SOFR lands at roughly 4.00% in 2031 with a 2.10% margin, producing a fully-indexed rate of 6.10% at first adjustment. Every number in the findings table is path-dependent. If SOFR is higher, the ARM's lifetime advantage shrinks or flips. Always run your own stress-test scenarios (index +1%, +2%, +3%) before committing.
Looking for a decision framework rather than the mechanics first? Read the fixed versus ARM suitability guide.
| 30-year fixed | 5/1 ARM | Takeaway | |
|---|---|---|---|
| Starter P&I (month 1) | $2,335 | $2,135 | $200/month lower on ARM |
| Total interest (modeled life) | $480,583 | $408,705 | $71,878 less on ARM path |
| Total cost over 30 years (P+I) | $840,583 | $768,705 | $71,878 less on ARM path |
US context
Fannie Mae's monthly housing outlook shows ARM originations typically double during periods when fixed rates exceed 7%, then fall as spreads compress. That national pattern does not dictate your decision, but it signals when lenders are actively competing on ARM pricing and when the initial rate discount is most meaningful.
Source: Fannie Mae, "Economic and Housing Outlook," accessed April 2026. URL: https://www.fanniemae.com/research-and-insights/forecast
When this modeling approach applies, and when it does not
Use it when your Loan Estimate shows an ARM and you want to see exactly what post-adjustment payments look like before you sign, with realistic cap assumptions in place.
Skip the ARM tabs when you have already chosen a fixed rate and only need taxes and insurance budgeting, or when your loan is HELOC-based with draw-phase mechanics this mortgage tool does not mirror.
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Frequently asked questions
- What inputs matter most when I model an ARM in a mortgage calculator?
- Capture the start rate, index, margin, initial fixed years, adjustment frequency, and every cap (first adjustment, periodic, lifetime). Without caps, a model can look unrealistically cheap or expensive.
- What is payment shock on an ARM?
- It is the jump in P&I after the fixed period when the fully indexed rate lifts the payment. Calculators should show the timing and size if you program index forecasts honestly.
- Should I assume the index stays flat forever?
- Flat indexes are a scenario, not a forecast. Stress the index up one or two points in a second tab to bracket worst-case payments inside your caps.
- Does this replace advice from a loan officer?
- No. It helps you ask better questions. Verify every assumption against your Loan Estimate figures.