Fixed vs. Adjustable-Rate Mortgages: How ARMs Work and When the Bet Pays
An ARM trades a lower starting rate for uncertainty later. We modeled one loan under three rate paths against a fixed loan to show when the ARM comes out ahead and when it doesn't.
A fixed-rate mortgage keeps the same rate for the life of the loan. An adjustable-rate mortgage (ARM) starts with a rate that is usually lower, holds it for an introductory period, and then adjusts periodically based on a market index. The trade is straightforward: a cheaper start in exchange for uncertainty later.
Whether that trade works out depends on two things: what rates do after the intro period, and how long you keep the loan. This guide explains how ARMs are built, then runs one hypothetical loan through three rate scenarios against a fixed-rate loan.
The short version
- An ARM's rate is fixed for an initial period (commonly 3, 5, 7, or 10 years), then adjusts at set intervals.
- After the intro period, the rate equals an index plus a margin, limited by caps.
- Caps limit how much the rate can change at the first adjustment, at each later adjustment, and over the life of the loan.
- In our hypothetical, the ARM saved interest through year 7 in every scenario, even when rates rose to the maximum. By year 10, the rising-rate scenario cost about $37,500 more interest than the fixed loan.
- ARMs tend to suit people who expect to sell or refinance before or shortly after the intro period, and who could afford the worst-case payment.
How an ARM is built
Initial period. A loan described as "5/1" traditionally means fixed for five years, then adjusting every year. Many newer ARMs adjust every six months after the intro period (for example, "5/6"). The loan documents spell out the schedule.
Index. A published market rate the loan follows. The specific index is named in the note.
Margin. A fixed number of percentage points added to the index. If the index is 4% and the margin is 2.75%, the fully indexed rate is 6.75%. The margin does not change.
Caps. Usually written as three numbers, such as 2/2/5:
| Cap | Meaning in a 2/2/5 structure |
|---|---|
| Initial adjustment cap | The rate can move at most 2 points at the first adjustment |
| Periodic cap | At most 2 points at each later adjustment |
| Lifetime cap | Never more than 5 points above the starting rate |
Some ARMs also have a floor, a minimum rate that applies even if the index falls a long way.
Your Loan Estimate shows the maximum possible payment and when it could occur, and lenders must provide disclosures explaining how your specific ARM adjusts. Read them.
The worked example
A hypothetical $400,000 loan, 30-year amortization.
- Fixed: 6.75% for 30 years.
- 5-year ARM: starts at 5.75%, adjusts annually after year 5, caps 2/2/5, so the maximum rate is 10.75%.
Three made-up paths for the ARM after year 5:
- Rates fall: resets to 5.25% and stays there.
- Rates flat: resets to 6.75%, the same as the fixed rate.
- Rates rise to the cap: 7.75% in year 6, 9.75% in year 7, 10.75% from year 8 on.
At each adjustment, the payment is recalculated so the remaining balance pays off over the remaining term.
Monthly payment
| Years 1–5 | Year 6 | Year 7 | Year 8+ | |
|---|---|---|---|---|
| Fixed at 6.75% | $2,594 | $2,594 | $2,594 | $2,594 |
| ARM, rates fall | $2,334 | $2,224 | $2,224 | $2,224 |
| ARM, rates flat | $2,334 | $2,564 | $2,564 | $2,564 |
| ARM, rates rise to cap | $2,334 | $2,803 | $3,294 | $3,545 |
In the worst case, the payment rises about $1,211 a month from the starting payment within three years of the first adjustment. That is the number to stress-test against your budget.
Cumulative interest paid
Show the numbers behind this chart
| Fixed 6.75% | ARM, rates fall | ARM, rates flat | ARM, rates rise to cap | |
|---|---|---|---|---|
| After 5 years | $131,166 | $111,106 | $111,106 | $111,106 |
| After 7 years | $181,079 | $149,318 | $160,427 | $175,194 |
| After 10 years | $252,531 | $203,440 | $231,031 | $290,037 |
Reading across:
- After 5 years, every ARM scenario has paid about $20,000 less interest than the fixed loan. That head start is what you buy with an ARM.
- After 7 years, even the worst-case ARM is still about $5,900 ahead of the fixed loan, because of the early savings.
- After 10 years, the worst-case ARM has paid about $37,500 more interest than the fixed loan, while the falling-rate ARM is about $49,000 ahead.
This is why the time horizon matters so much. An ARM's advantage is front-loaded. Its risk is back-loaded.
When ARMs tend to make sense
- You expect to move within or shortly after the intro period, such as a planned relocation or a starter home.
- You expect higher income later that would absorb a payment increase.
- The rate gap is large between ARM and fixed rates, which makes the early savings bigger.
- You could afford the worst-case payment without hardship. This one is not optional.
When fixed tends to make sense
- You plan to stay a long time and value certainty.
- Your budget is tight, so a payment increase would be a real problem.
- The rate gap is small, so the ARM's head start is minor.
- You don't want to depend on refinancing, which requires qualifying again and depends on market conditions at that time.
The refinancing assumption
Many ARM borrowers plan to refinance before the first adjustment. That plan can fail: rates may be higher, home values may have fallen, income may have changed, or lending standards may have tightened. Treat refinancing as an option, not a certainty, and be sure you could carry the adjusted payment if it doesn't happen.
Questions to ask about any ARM
- What index does it follow, and what is the margin?
- What are the initial, periodic, and lifetime caps? Is there a floor?
- How often does it adjust after the intro period?
- What is the maximum possible payment, and when could it occur?
- Are there prepayment penalties?
- Can it be converted to a fixed rate, and at what cost?
The takeaway
An ARM buys a lower rate for the first few years in exchange for risk later. If you are likely to sell or refinance within the intro period and could handle the worst-case payment, it can save real money. If you plan to stay for a long time or your budget has no slack, a fixed rate removes the guesswork. Model the worst case with your actual loan terms before deciding.
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