Fixed-Rate vs. Adjustable-Rate Mortgage (ARM): What's the Difference?
Last updated September 2026
Quick answer
A fixed-rate mortgage locks your rate for the entire loan. An ARM (adjustable-rate mortgage) offers a lower rate for an initial period — 5, 7, or 10 years — then adjusts periodically based on a market index. On a $400,000 loan, a 5/1 ARM at 5.75% saves about $129/month ($7,713 over 5 years) versus a 6.25% fixed loan. But if the rate rises to its cap at the first adjustment, the new payment could jump to about $2,802/month — $339 more than the fixed-rate borrower is paying by then. The savings are real; so is the risk.
How ARM naming works
The two numbers tell you everything: the first is how many years the rate is fixed, the second is how often it adjusts after that.
- 5/1 ARM — fixed 5 years, then adjusts once a year
- 7/1 ARM — fixed 7 years, then adjusts once a year
- 10/1 ARM — fixed 10 years, then adjusts once a year
After the fixed period, your new rate is set by an index (almost always SOFR now, the Secured Overnight Financing Rate) plus a fixed margin your lender sets at closing, typically around 2.5%–3%. As the index moves, your rate moves — within limits.
The example we'll use throughout
| 30-year fixed | 5/1 ARM | |
|---|---|---|
| Loan amount | $400,000 | $400,000 |
| Rate (years 1–5) | 6.25% | 5.75% |
| Monthly P&I | $2,462.87 | $2,334.32 |
| Monthly savings | — | $128.55 |
| Savings over 5 years | — | $7,713 |
Rate caps: what limits the adjustment
Modern ARMs are capped, written as three numbers — e.g., 2/2/5:
- First cap (2%) — the most the rate can move at the first adjustment
- Periodic cap (2%) — the most it can move at each adjustment after that
- Lifetime cap (5%) — the most it can ever rise above your starting rate
On our 5.75% ARM with 2/2/5 caps, the rate can never exceed 10.75% — but it doesn't need to get anywhere near that to hurt. Here's the worst-case first adjustment:
| ARM, year 5 (before adjustment) | ARM, year 5 (worst-case adjustment) | Fixed, year 5 | |
|---|---|---|---|
| Rate | 5.75% | 7.75% (+2%, at cap) | 6.25% (unchanged) |
| Remaining balance | $371,061 | $371,061 | $373,352 |
| Monthly payment | $2,334.32 | $2,802.14 | $2,462.87 |
If the rate adjusts all the way to the cap, the ARM payment jumps by $467.82 in a single month — and lands $339 above what the fixed-rate borrower is paying, after years of paying less. This is the scenario an ARM decision has to be able to survive, not the average case.
Things to consider
- Your exit plan matters more than the rate. ARMs make the most sense when you have a real reason to expect you'll sell or refinance before the fixed period ends — a planned move, a known relocation, an intentionally short hold.
- You may have to qualify at the higher rate anyway. Many lenders underwrite ARMs using the fully-indexed or capped rate, not the low intro rate — so the lower payment doesn't always translate into qualifying for a bigger loan.
- Caps vary by loan. 2/2/5 is common, but some loans use different structures (e.g., 5/2/5 on 7- and 10-year initial periods) with a larger first jump. Confirm your exact caps before signing.
- Rates can also fall. If the index is lower than your margin implies at adjustment, your ARM payment can actually drop — the risk runs both directions, even if the downside gets more attention.
- No prepayment penalty, generally. Modern U.S. ARMs typically let you refinance or sell penalty-free, which is exactly how most ARM borrowers plan to exit before the adjustment risk becomes real.
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Frequently asked questions
What does "5/1 ARM" mean?
The rate is fixed for the first 5 years, then adjusts once a year after that, based on an index plus your lender's margin.
How much can an ARM rate increase?
It depends on the caps. A common structure is 2/2/5: up to 2% at the first adjustment, up to 2% at each adjustment after that, and up to 5% total above your starting rate over the life of the loan.
Is an ARM riskier than a fixed-rate mortgage?
Yes, in the sense that your payment isn't guaranteed to stay the same after the intro period. Caps limit how much it can move at once, but over the life of the loan it can still rise substantially — as shown above, a single capped adjustment can wipe out years of savings.
Do I have to qualify at the higher potential rate?
Often, yes. Many lenders underwrite ARMs against the fully-indexed or capped rate rather than the low introductory rate, which limits how much the lower payment actually helps you qualify for.
Can I refinance out of an ARM before it adjusts?
Yes — this is the most common ARM strategy. Take the introductory savings, then refinance to a fixed rate (or sell) before the first adjustment. Modern U.S. ARMs generally carry no prepayment penalty.
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Read moreFigures on this page are generated from the same amortization engine that powers the MortMetrix dashboard, using the example loan stated in each table. They are estimates based on a fixed-rate loan at a constant rate and are not your actual loan terms. This is educational information, not financial advice.