Fixed vs Adjustable-Rate Mortgage: Which One Wins?
A fixed-rate mortgage locks one payment for the life of the loan; an adjustable-rate mortgage (ARM) starts 0.5–1 point cheaper and can rise after an initial fixed period. On a $350,000 loan, a 6.6% fixed rate costs $2,235 a month while a 5/1 ARM at 5.9% costs $2,076 — but if that ARM resets to 7.9% after five years, the payment jumps to about $2,489. Choose the ARM only if you will exit within the fixed window or can absorb the capped maximum payment.
The real question is not which rate is lower today — it is who carries the risk that rates move.
The core trade: with a fixed rate, the lender carries interest-rate risk and charges you for it through a higher rate. With an ARM, you carry the risk after the introductory period and are paid for it through a lower start rate. The right choice is decided by two numbers — how long you will keep the loan, and whether you could still pay it at the ARM's lifetime cap.
Not financial advice. This guide is for information only. It is not a recommendation to borrow or choose any loan product. Read the full disclaimer.
How each structure works
A fixed-rate mortgage never changes: the 6.6% you sign is the 6.6% you pay in year 25. The payment is fully amortizing, meaning every payment retires some principal, and the schedule is knowable to the cent on day one (see the amortization definition).
An adjustable-rate mortgage is really two loans in sequence. A "5/1 ARM" fixes the rate for five years, then adjusts once a year (a 5/6 adjusts every six months). Each adjustment sets the new rate as a published index — usually SOFR — plus a fixed margin, often 2.5–3%. Three caps bound the damage: an initial cap on the first adjustment (commonly 2%), a periodic cap on each later change (commonly 1–2%), and a lifetime cap over the start rate (commonly 5%). A 5.9% ARM with 2/2/5 caps can reach 7.9% at year six and can never exceed 10.9%.
Side-by-side on a $350,000 loan
The table compares a 30-year fixed at 6.6% with a 5/1 ARM at 5.9% (2/2/5 caps, SOFR + 2.75%), both fully amortizing, principal and interest only. Figures are exact to the cent from the standard amortization formula — rerun your own scenario in the mortgage calculator.
| Feature | 30-year fixed, 6.6% | 5/1 ARM, 5.9% start |
|---|---|---|
| Monthly payment, years 1–5 | $2,235.31 | $2,075.98 |
| Balance after 5 years (60 payments) | $327,650 | $325,285 |
| Payment if ARM resets to 7.9% (year 6) | Still $2,235.31 | $2,489.09 |
| Worst-case payment (lifetime cap 10.9%) | $2,235.31 | ~$3,060 |
| Total paid if held 30 years (no reset) | $804,712 | $747,353 |
| Rate risk after year 5 | None | Adjusts every 6–12 months |
Three things to notice. First, the ARM saves $159 a month during the fixed window — about $9,560 over five years, plus a slightly lower balance. Second, a single full-cap reset erases those savings in about two years and leaves you paying $254 a month more than the fixed borrower. Third, "no reset" is an assumption, not a promise: the ARM only behaves like the cheap option if you exit or rates stay flat.
Three borrower scenarios
The mobile buyer (wins with the ARM). A relocating professional who expects to sell within four to five years collects the entire $9,560 of savings and exits before the rate ever moves. The ARM is the rational choice — as long as the plan survives a delay, which is why the worst-case payment must still be affordable.
The 30-year owner (wins with the fixed). A family planning to stay until the loan is gone pays for certainty and receives it. If rates fall, they can refinance; if rates rise to 9%, the payment stays $2,235.31. Over 30 years the fixed borrower in our table pays $804,712 against an unknown ARM total bounded only by the caps — potentially well above $1 million near the lifetime maximum.
The stretched buyer (should take neither at the maximum). If you only qualify because the ARM's start rate is lower, the loan is telling you the house is too expensive. Lenders typically qualify ARM borrowers at the start rate or a modest buffer above it; if you cannot carry the year-six payment of $2,489 in our example, the ARM is not a discount — it is a delay. Check the sustainable payment first in our affordability guide.
The break-even test before you choose
Ask two questions with numbers, not feelings. One: how long will I realistically keep this loan? The median US homeowner sells within about 13 years, but job moves, family changes, and refinancing make individual horizons shorter — if yours is under the ARM's fixed period with margin to spare, the ARM's discount is largely free money. Two: can I pay at the cap? Compute the payment at start rate + lifetime cap (about $3,060 a month on our example) and check it against your budget. If it fails, take the fixed rate or a smaller loan.
Also compare like for like on cost: an ARM with a lower rate but a point of origination fees can cost more than a no-points fixed loan. Compare APRs, not headline rates — our APR calculator folds the fees into one comparable number, and the refinance calculator shows what a future exit would cost. For the earlier fixed vs variable overview see fixed vs variable rate.
Frequently Asked Questions
Is a fixed or adjustable rate better right now?
It depends on how long you will keep the loan and whether you can absorb the worst case. If you expect to sell or refinance within the ARM's fixed period (typically 5–7 years) and the start-rate discount is meaningful, an ARM usually wins. If you will stay longer, or the payment at the ARM's maximum cap would strain your budget, the fixed rate is the safer choice.
How often can an ARM rate change?
After the initial fixed period, most ARMs adjust every six or twelve months. Caps limit each change — a common structure is 2% at the first adjustment, 2% per subsequent adjustment, and 5% over the life of the loan — so a 5.9% start rate with a 5% lifetime cap can never exceed 10.9%.
Can I refinance out of an ARM before it adjusts?
Yes, if you still qualify — meaning adequate equity, income, and credit at that future date — and you will pay closing costs of roughly 2–3% of the balance. Treat refinancing as an option, not a plan: if rates rise or your home value falls, the exit may not be available on good terms.