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Fixed vs Adjustable-Rate Mortgages: Risks Compared

Reviewed by Unpack Money Research Desk · Updated October 28, 2026 · Our review process

Quick answer

Fixed-rate mortgage: the interest rate — and therefore the principal-and-interest payment — never changes for the life of the loan. Adjustable-rate mortgage (ARM): the rate is fixed for an initial period, then adjusts periodically based on an index plus a margin, subject to caps. ARMs typically start below comparable fixed rates. That discount is the price of accepting that your payment can rise later. The question is never 'which rate is lower today?' but 'what happens to my payment, and my budget, if rates rise — and can I still afford the loan at its capped maximum?'

Certainty costs money; a discount carries reset risk

Fixed-rate mortgage: the interest rate — and therefore the principal-and-interest payment — never changes for the life of the loan.

Adjustable-rate mortgage (ARM): the rate is fixed for an initial period, then adjusts periodically based on an index plus a margin, subject to caps.

ARMs typically start below comparable fixed rates. That discount is the price of accepting that your payment can rise later. The question is never "which rate is lower today?" but "what happens to my payment, and my budget, if rates rise — and can I still afford the loan at its capped maximum?"

How an ARM actually works

After the initial fixed period (commonly 5, 7, or 10 years in products labeled 5/6, 7/6, 10/6 — the second number indicating adjustment frequency — though structures vary), the rate becomes: New rate = Index value + Margin.

The margin is fixed for the life of the loan; the index moves with the market. Three caps bound the movement, and all three must be confirmed for the specific product — they are not standard across ARMs:

Initial adjustment cap: how much the rate can move at the first adjustment.

Periodic cap: how much it can move at each later adjustment.

Lifetime cap: the maximum rate the loan can ever reach.

Worked logic (illustrative, no product quoted): an ARM starting at an illustrative 6.00% with a lifetime cap 5 points above the start could reach 11.00%. Before signing any ARM, compute the payment at that lifetime-cap rate on your loan amount. If that payment breaks your budget, the ARM's initial discount is not worth the risk — regardless of how likely a rise seems.

Worked example: the fixed-rate anchor

At the October 22, 2026 PMMS averages, a $400,000 loan's principal-and-interest payment was about $2,748 per month over 30 years at 7.32% fixed, and about $3,524 over 15 years at 6.68% fixed. A fixed payment lets you budget that figure for the entire term. An ARM's payment, by contrast, is only knowable through its initial period; everything after is a range bounded by caps, not a number. Compare an ARM quote against the fixed payment you would otherwise hold for decades, not against the ARM's first-year payment alone. Dated benchmark illustration (October 22, 2026 snapshot — check the latest release for current figures).

The refinance assumption — and why it fails

Many ARM borrowers plan to refinance or sell before the first adjustment. That plan fails if: rates are higher at reset, home values fall (reducing equity and refinance eligibility), income falls, credit deteriorates, or the property becomes hard to sell. An ARM is only prudent if you can afford the loan at its capped rate and still want it if the refinance plan fails. If affordability depends on refinancing on schedule, you cannot afford the ARM's downside.

Decision checklist

1. How long will I certainly keep this loan — and how certain is "certainly"?

2. What is the payment at the lifetime-cap rate, and can my budget absorb it indefinitely?

3. What are the initial, periodic, and lifetime caps, the index, the margin, and any floor — for this exact product?

4. Does the ARM have a conversion option to fixed, and what does it cost?

5. Am I choosing the ARM for its discount, or because the fixed payment is unaffordable? If the fixed payment is unaffordable, the answer may be a smaller loan, not a riskier one.

Disclosure and disclaimer

Disclosure: Unpack Money currently earns $0 in affiliate revenue and has no approved affiliate programs. No lender is recommended and no affiliate links appear here.

Disclaimer: Educational only — not financial advice and not a mortgage offer. ARM terms vary by product; verify the index, margin, caps, and adjustment schedule in writing before deciding. This page states no ARM-specific figure (initial rate, margin, index, periodic/lifetime caps, fixed-period length) and no claim about future rate direction.

At a glance

Fixed-rate vs adjustable-rate mortgage

What this means for you

Budget the ARM at its lifetime-cap payment, not its first-year payment. If that capped payment breaks your budget — or if affordability depends on refinancing on schedule — the fixed payment, or a smaller loan, is the honest answer.

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