What Is an ARM Mortgage? — Plain-English Guide
If you have ever wondered why a 5/1 ARM is cheaper than a 30-year fixed and what happens after year 5, this page is for you. No jargon without a definition. No formulas without an example.
Acronym decoder
You will see these terms in every ARM disclosure. Skip the ones tagged "you don't need to know this yet" until you are shopping for a real loan.
| Acronym | Stands for | Plain-English meaning |
|---|---|---|
| ARM | Adjustable-Rate Mortgage | A mortgage whose rate changes over time. |
| SOFR | Secured Overnight Financing Rate | The index most ARMs use today. Published daily by the NY Fed. |
| LIBOR | London Interbank Offered Rate (retired) | Legacy index used before 2024. You don't need to know this yet. |
| Margin | (no acronym) | The lender's fixed markup added to the index. Does not change. |
| Cap | (no acronym) | A limit on how much the rate can adjust. |
| Floor | (no acronym) | The minimum rate the loan can reach. Usually equal to the margin. |
| MTA | 12-Month Treasury Average | A smoothed index used in some low-volatility ARMs. You don't need to know this yet. |
| COFI | 11th District Cost of Funds Index | A legacy western-state index. Rare in new originations. |
| GPM | Graduated Payment Mortgage | A different product where payments start low and rise on a schedule. Not an ARM. You don't need to know this yet. |
Anatomy of a 5/1 ARM, label by label
When you see "5/1 ARM at 5.25% with 5/2/5 caps and a 2.75% margin," here is what every piece means.
- "5" (the first digit): your rate is fixed for 5 years. Nothing changes during this period.
- "1" (the second digit): after the fixed period, your rate adjusts once per year. A 5/1 ARM has 25 potential adjustments (years 6-30). A 5/6 ARM would adjust every 6 months.
- "5.25%" (the start rate): the interest rate during the fixed period. This is what you will pay for the first 5 years.
- "5/2/5 caps": the first 5 is the initial cap (the rate can move at most 5% at the first adjustment). The 2 is the periodic cap (the rate can move at most 2% at any subsequent adjustment). The last 5 is the lifetime cap (the rate can never be more than 5% above the start rate).
- "2.75% margin": when the rate adjusts, the lender adds 2.75% to the current index value. If SOFR is 4.30% at your first adjustment, your fully indexed rate is 7.05%. The cap then limits the actual move.
After year 5, with the assumptions above and SOFR at 4.30%, your rate moves from 5.25% to 7.05% — but the initial 5% cap kicks in, so the actual rate moves to 7.25% (5.25% + 2%, the smaller of the cap-bound move and the index-bound move). The cap binds first.
A short history: why ARMs changed after 2008
Before 2008, most ARMs used LIBOR as their index. LIBOR was a survey-based rate that banks reported to each other — and during the financial crisis it was revealed to be manipulated. LIBOR was phased out by 2023 and fully retired in 2024.
Modern ARMs use SOFR, which is transaction-based (calculated from actual overnight Treasury-backed lending). SOFR is harder to manipulate because it is grounded in real trades, not surveys. For borrowers, the practical difference is mostly cosmetic: an ARM tied to SOFR behaves similarly to a LIBOR ARM, but with a different number behind the index value.
Is an ARM right for me? A 4-question self-check
Before shopping for an ARM, walk through these. If you cannot answer "yes" to all four, an ARM is probably not your product.
- Do I know my move date? ARMs work when paired with a real exit. "I'll move in 5 years" is a guess. "My job assignment ends in March 2029" is a date.
- Can I afford a 50% higher payment? With a 5/2/5 cap on a $400k loan at 5.25%, the worst-case adjusted payment is $2,830/month — a 28% increase. For a 10/1 ARM at 6.00%, the worst case is more. If your household budget cannot absorb that without sacrificing other essentials, the ARM is too risky.
- Do I understand my rate caps? If you cannot explain what an initial cap, periodic cap, and lifetime cap are, you cannot evaluate an ARM offer. Read the caps disclosure before signing.
- Am I getting an ARM because I want one, or because someone pushed me toward one? Loan officers earn the same commission on most products. If your loan officer is enthusiastically recommending an ARM without explaining the payment-shock risk, ask why.
Where to go from here
Now that you know what an ARM is, see ARM vs Fixed-Rate Mortgage for the decision framework and How ARM Payments Are Calculated for the mechanics. The ARM Calculator will let you model any initial rate, margin, and cap structure to see your year-by-year payment path.