Fixed-Rate vs Adjustable-Rate Mortgages
Fixed-Rate vs Adjustable-Rate Mortgages
A fixed-rate mortgage locks in certainty for 15 or 30 years; an ARM trades a lower initial rate for the risk that rates will reset higher. ARMs make sense only if you're confident you'll refinance or sell before reset.
Key takeaways
- Fixed-rate mortgages (30-year or 15-year) lock the interest rate for the entire loan term, providing payment certainty and simplicity.
- ARMs (adjustable-rate mortgages) start with a low "teaser" rate, then reset to market rates after a fixed period (5/1, 7/1, 10/1 ARMs).
- A 5/1 ARM at 5.5% (instead of 6.5% fixed) saves $100–$150/month initially but risks resetting to 7.5%–8.5% in year 6.
- ARM rate caps (periodic and lifetime) limit how much rates can jump per reset, but don't prevent significant payment shocks.
- ARMs are dangerous if you plan to stay long-term (10+ years) or if you can't refinance when rates reset.
Fixed-rate mortgages: The standard
A fixed-rate mortgage charges the same interest rate for the entire 15, 20, or 30-year term. Your monthly payment never changes (excluding escrow adjustments for taxes and insurance).
30-year fixed:
- Lowest monthly payment (interest spread over longest period).
- Predictable cost for lifetime of the loan.
- Easy to budget and refinance.
- Rate is typically 0.5–0.75% higher than a 5/1 ARM.
15-year fixed:
- Monthly payment is 20–25% higher than 30-year.
- Loan is paid off in half the time; total interest paid is often 40% lower.
- Builds equity faster; useful for retirement planning.
- Rate is typically 0.25–0.5% lower than 30-year fixed.
Example on a $280,000 loan at 6%:
| Term | Monthly P&I | Total paid over term | Total interest |
|---|---|---|---|
| 30-year | $1,679 | $604,440 | $324,440 |
| 15-year | $2,110 | $379,800 | $99,800 |
Difference: The 15-year costs $431/month more but saves $224,640 in interest. For those with stable income and no other competing priorities (maxing 401k, paying off high-interest debt), a 15-year is mathematically superior.
Adjustable-rate mortgages: The teaser
An ARM starts with a lower rate (the "teaser" rate) for a fixed period, then resets to a market-based rate. A 5/1 ARM means 5 years at the teaser rate, then annual resets for the remaining 25 years.
Common ARM structures:
- 5/1 ARM: 5 years fixed, then adjusts annually
- 7/1 ARM: 7 years fixed, then adjusts annually
- 10/1 ARM: 10 years fixed, then adjusts annually
- 3/1 ARM (rare now, popular during 2000s): 3 years fixed, then adjusts annually
Rate reset mechanics: After the fixed period, the rate resets based on an index (typically SOFR or LIBOR) plus a lender margin (usually 2–2.5%). If SOFR is 4% and margin is 2%, the new rate is 6%.
Rate caps:
- Periodic cap: Maximum increase per adjustment, typically 1–2% per year.
- Lifetime cap: Maximum increase over the life of the loan, typically 5–6% above the original rate.
Example: 5/1 ARM at 5.5% with 1% annual cap and 5% lifetime cap.
- Years 1–5: 5.5% (fixed)
- Year 6: Can't exceed 5.5% + 1% = 6.5%
- Year 7: Can't exceed 6.5% + 1% = 7.5%
- Year 10: Could reach 10.5%, but lifetime cap caps it at 5.5% + 5% = 10.5%
ARM math: The payment shock
Suppose you take a 5/1 ARM on a $280,000 loan at 5.5% initial rate (vs. 6.5% for a 30-year fixed).
Years 1–5:
- Monthly P&I: $1,592 (ARM)
- Monthly P&I (30-year fixed, 6.5%): $1,779
- Monthly savings: $187
Year 6 (rate resets to 7.5%):
- New monthly P&I: $1,956 (based on remaining principal of ~$230,000 at 7.5% over 25 years)
- Payment jump: $1,956 − $1,592 = $364/month increase (23% jump)
Over 5 years, you saved $187 × 60 = $11,220. In year 6, a single $364 jump erases that entire savings and leaves you worse off if rates stay high.
Year 7 (rate resets to 8.5%):
- New monthly P&I: $2,142
- Total jump from year 5: $550/month (35%)
At this point, your ARM costs more than the fixed-rate loan would have—and for the remaining 23 years, you're locked into a market-rate mortgage that could exceed historical averages.
When ARMs made sense (and when they don't)
ARMs were rational in the 1990s–early 2000s when:
- Rates were high (7–8%) and expected to fall
- Home values were appreciating rapidly
- Borrowers had strong income growth
- Refinance windows were clear
ARMs are riskier now (2024+) because:
- Rates are already elevated (5–7% range), and further increases are possible
- Home appreciation is slower and less certain
- Income growth has stalled in many fields
- Refinance risk is higher if you can't qualify when rates spike
ARM scenarios: When you might win
Scenario 1: Planned 7-year hold, refinance before reset
You buy with a 7/1 ARM at 5.75% instead of 6.5% fixed. You plan to refinance at year 7, before the first reset. If rates have fallen to 5.5%, you refinance to a 30-year fixed at 5.5% and lock in a gain. You saved $187/month for 7 years ($15,680 total) and refinance into a lower rate.
Risk: Rates don't fall; they stay at 6%+ or rise. You can't refinance, and you're stuck with a reset from 5.75% to 7%+.
Scenario 2: Strong income growth, confident in higher payment
You're a 28-year-old engineer with bonus income and stock options. You expect your income to double in 10 years. You take a 7/1 ARM at 5.75% initially costing $1,592/month. In year 7, rates reset to 7%, payment jumps to $1,956 (now 25% of gross income instead of 15%). You can absorb it. Mathematically, you come out ahead.
Risk: You're laid off, bonus evaporates, or stock devalues. Suddenly a $1,956 payment is unaffordable.
ARM scenarios: When you lose
Scenario 1: Planning to stay 10+ years, no refinance window
You take a 5/1 ARM at 5.5%, intending to stay in the home 20 years. Year 6 arrives, rates have risen to 8%, and your payment jumps $400/month. You can't or won't refinance. For the remaining 24 years, you're paying above market, eating opportunity cost.
Scenario 2: Rates rise during initial period, reset is massive
You take a 3/1 ARM at 4.5% in 2021. By year 4 (2025), rates have jumped to 6.5%+. Year 4 reset hits 6.5%, a 2% jump. If you have a 2% annual cap, payment jumps, then another 2% jump year 5, then another. Three years of resets at the cap could take you to 8.5%+ by year 7. Your ARM savings evaporate entirely.
This happened broadly 2022–2024; millions of borrowers with 3/1 and 5/1 ARMs issued in 2020–2021 saw massive payment shocks.
ARM vs. fixed: A break-even framework
An ARM makes sense if:
- You'll definitely sell or refinance before the first reset (and rates are expected to fall or stay low).
- You have substantial job/income security and can absorb a 30%+ payment jump.
- The rate discount is large (1%+ vs. fixed), so you have time to recover savings.
An ARM does not make sense if:
- You plan to stay 10+ years.
- Job stability is uncertain.
- You're already stretching your DTI ratio (over 35%).
- Rates are historically high and may go higher.
Worked example: ARM vs. fixed decision
Loan: $280,000. Down payment: 20% on $350,000 home.
Option A: 30-year fixed at 6.5%
- Monthly P&I: $1,779
- 30-year total: $640,440 (total interest: $360,440)
- Certainty: Total cost is locked in
Option B: 5/1 ARM at 5.5%, resets to 7.5% in year 6
- Years 1–5: $1,592/month
- Year 6 onwards: ~$1,956/month (based on remaining balance)
- 30-year total: ~$630,000 (similar to fixed due to reset)
- Uncertainty: Payment and cost depend on market rates
If rates fall to 5% by year 6, an ARM refinance locks you in at 5% and you gain $1,779 − $1,505 = $274/month. If rates rise to 8%, a reset is $2,265/month—$486 worse than fixed.
Probability-weighted: If you think 50% chance rates fall, 50% stay high, expected value is near-identical. But downside risk (rates rise) is asymmetric; your payment could spike 30%, while fixed-rate upside is capped. This asymmetry makes fixed-rate the safer bet for most borrowers.
Decision tree
Next
You've chosen a fixed or ARM structure. Now let's understand how the mortgage is paid off—why early payments are mostly interest, and why a 30-year mortgage costs so much more than a 15-year. Amortization is the silent driver of total cost.