Fixed vs Adjustable Rate Calculator

Compare fixed-rate mortgages with adjustable-rate mortgages (ARM).

Status: Algorithmic Audit (CPA Review Pending)
Jurisdiction: United States (IRS & SSA Standards)
Tax Year / Constants: 2026 Statutory Limits
Data Privacy: 100% Client-Side In-Browser

The Mathematical Formula

Payment Differential = Monthly Fixed Payment - Monthly ARM Payment ; Break-Even Horizon is when Cumulative ARM Payments exceed Fixed Payments after Rate Resets

Variable & Parameter Definitions

  • Monthly Fixed Payment Guaranteed, unvarying 30-year fixed principal and interest payment.
  • Monthly ARM Payment Discounted teaser payment during the initial locked period (e.g. Years 1–5 or 1–7).
  • Upfront Cash Savings Total monthly cash savings banked during the discounted initial fixed teaser period.
  • Break-Even Horizon The future month where potential ARM rate spikes erase all previously accumulated upfront savings.

Real-World Worked Scenario: 30-Year Fixed at 6.75% vs 7/1 ARM at 5.75% on $450,000

A buyer borrows $450,000 comparing a 30-year fixed loan at 6.75% ($2,918/mo) versus a 7/1 ARM at 5.75% ($2,626/mo).

  1. Monthly Teaser Savings: $2,918 - $2,626 = $292 per month saved.
  2. Guaranteed 7-Year Accumulation: $292/month × 84 months = $24,528 in guaranteed cash savings during Years 1 to 7.
  3. Year 8 Reset Scenario: If rate adjusts to 7.75% ($3,090/mo), the ARM payment is +$172/month higher than the fixed loan.
  4. Catch-Up Horizon: $24,528 banked savings ÷ $172 monthly difference = 142.6 months (11.8 years after reset, or Year 19 of ownership).
  5. Decision Takeaway: If the buyer plans to move within 7 to 10 years, the 7/1 ARM delivers massive guaranteed financial savings.
Key Financial Takeaway: An ARM provides guaranteed initial savings. The risk only manifests if you remain in the property long after the initial fixed period expires during high interest rate environments.

Strategic Insights & Pitfalls to Avoid

  • ✓ Match Loan to Holding Period: If you plan to live in the home for 5 to 7 years (typical first-time homebuyer duration), a 7/1 ARM eliminates rate risk during your entire ownership period.
  • ✓ Bank the Difference: Deposit the monthly payment savings into an emergency fund or index fund rather than spending it.
  • ✓ Refinance Optionality: If rates drop within the first 5 to 7 years, you can refinance your ARM into a permanent low fixed-rate loan.

Frequently Asked Questions

Is a fixed-rate mortgage always safer than an ARM?

Fixed loans provide 100% payment certainty. However, you pay a premium for that insurance in the form of a higher interest rate throughout the early years.

What happens if interest rates crash while I have an ARM?

Your ARM payment automatically resets downward at the annual adjustment date without paying refinance closing costs.

Can an ARM payment double overnight?

No. Statutory and contractual adjustment caps (typically 2% per year) prevent sudden catastrophic payment surges.

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