What is Fixed vs ARM Cost Comparison?
A fixed-rate vs adjustable-rate mortgage (ARM) comparison calculates the total cost difference under different interest rate scenarios over 5-10 year periods. In the US, the 30-year fixed dominates at roughly 90% of originations, but ARMs can save thousands for buyers who plan to sell or refinance during the initial fixed period.
The Formula
Fixed Cost = Monthly Payment × Term Months ARM Cost = (Initial Fixed Period Monthly × Months) + (Adjustable Period Monthly at Projected Rate × Months)
Worked Example
$320,000 mortgage: 30-year fixed at 6.75% or 5/1 ARM at 5.75% (initial).
- Fixed monthly P&I: $2,076 × 60 months = $124,560
- ARM initial monthly P&I: $1,867 × 60 months = $112,020
- ARM 5-year savings if rate stays stable: $12,540
- If ARM adjusts up 2% at month 61 → $2,274/month thereafter
- Break-even: ARM saves as long as you sell or refinance within the 5-year fixed period
📌 ARM saves $12,540 over the first 5 years, but faces payment shock if rates rise and you stay past month 61. Fixed provides 30 years of payment certainty.
Why This Matters
Payment certainty
A 30-year fixed eliminates monthly payment surprise and rate-shock risk. For tight budgets and long-term owners, certainty is worth a premium.
Interest savings
ARMs are typically 0.5-1.0% lower than the 30-year fixed at origination per Freddie Mac PMMS. Over 5 years that saves $10,000-20,000 on a typical mortgage if rates remain stable.
Mobility match
A 5/1 or 7/1 ARM aligns with the median US homeowner tenure of 7-13 years per NAR data. If you know you will sell or refinance, the ARM discount is free money.
Common Mistakes
❌ Choosing based on initial rate only
The 5.75% ARM rate looks great vs the 6.75% fixed, but ARMs adjust based on SOFR + margin after the fixed period. Model scenarios where rates rise 2-5% at the lifetime cap.
❌ Ignoring discount points
A lower rate with 1.5 discount points may cost more than a slightly higher rate with zero points, especially if you plan to sell or refinance within 3-5 years. Compare breakeven months.
❌ Not considering life plans
If you might stay 10+ years, a 5/1 ARM with a lifetime cap 5% above start rate is a costly mistake. Match loan to planned hold period.
Industry Benchmarks
| Category | Good | Average | Poor |
|---|---|---|---|
| 30-Year Fixed | Below 6.25% | 6.25-7.0% | Above 7.25% |
| 15-Year Fixed | Below 5.5% | 5.5-6.25% | Above 6.5% |
| 5/1 ARM | Below 5.75% | 5.75-6.25% | Above 6.5% |
Source: Freddie Mac Primary Mortgage Market Survey (PMMS) 2026
Benchmark data sourced from Freddie Mac Primary Mortgage Market Survey (PMMS) 2026.