5/1 ARM vs 30-year fixed

The most common mortgage question after “how big a payment can I afford” is whether to take an adjustable-rate mortgage (ARM) for a lower initial rate, or lock in the predictability of a 30-year fixed. The answer is almost entirely about your time horizon.

How an ARM works

A 5/1 ARM has a rate fixed for 5 years, then adjusts annually. Common variants:

• 5/1 ARM: 5 yrs fixed, then adjusts every 12 months
• 5/6 SOFR ARM (modern): 5 yrs fixed, then adjusts every 6 months
• 7/1, 10/1: longer fixed periods, slightly higher initial rates

Adjustments are bounded by caps:

• Initial cap (typically 2%): max rate jump at the first adjustment
• Periodic cap (typically 2% on an annually adjusting 5/1, 1% on a 6-month-adjusting 5/6): max rate jump at each subsequent adjustment
• Lifetime cap (typically 5%): max total rate increase above your initial rate over the loan’s life

So a 5/1 ARM at 5.5% with 2/1/5 caps (a tighter periodic cap than most annual 5/1s) could go to 7.5% in year 6, 8.5% in year 7, and ultimately 10.5% over the life of the loan. With the more common 2/2/5 caps, it could reach 10.5% by year 8.

The tradeoff

ARM rates are lower than fixed rates because you bear the rate-reset risk instead of the lender. The discount on the initial rate is real, but it has a known shelf life.

Some quick numbers (illustrative, not current):

Loan	Initial Rate	Year 1 Payment ($300k)
30-yr fixed	6.50%	$1,896
5/1 ARM	5.75%	$1,751
7/1 ARM	6.00%	$1,799

Saving $145/month for 5 years = $8,700. Not nothing. But if rates rise and your year-6 payment jumps to $2,100 (the most a 2% first cap allows), the savings evaporate quickly.

When the ARM wins

• You’ll definitely move within the fixed period. Job moves, growing family, military relocation. If you’ll sell in years 3-4 of a 5/1 ARM, you captured the discount and never see the reset.
• You’ll definitely refinance within the fixed period. Rates are unusually high right now and you expect to refi when they drop. The ARM lets you save money in the meantime.
• You can comfortably afford the worst-case rate. The lifetime cap is the relevant stress test. A 5/1 ARM at 5.5% with a 5% lifetime cap could go to 10.5%. Could you still pay that? If yes, the ARM is fine. If no, you’re taking on tail risk you can’t absorb.

When the fixed wins

• You’re planning to stay 7+ years and rates may rise. The fixed loan locks in current pricing for 30 years. If you refi later, that’s your option, not an obligation.
• You can’t comfortably afford the ARM’s worst-case payment. Refusing this stress test is a common way to end up forced into a short sale during a rate spike.
• You value sleep over $145/month. The cognitive overhead of watching SOFR every six months is a real cost that ARM advocates underestimate.

Running the numbers

Use the Mortgage calculator to model both scenarios:

1. Set up the 30-year fixed scenario with today’s rate
2. Switch to ARM mode and enter the 5/1 details: initial rate, adjusted rate (your best guess at the fully-indexed rate after year 5), and the loan’s caps
3. Compare year-1 monthly payment, year-6 estimated payment, and total interest over a realistic hold period

The “expected fully-indexed rate” is your best guess at SOFR + your loan’s margin (typically 2.5–3%) at the time of the first reset. Check the current 30-day average SOFR (published daily by the New York Fed) and add your margin for today’s figure. Five years from now, who knows. That’s the risk you’re taking.

Common ARM mistakes

• Underestimating the reset. People look at today’s SOFR and assume future SOFR will be similar. Often it’s not.
• Planning to refi at year 5 without checking break-even. If rates are higher than your fixed-period rate, refinancing means re-pricing the entire balance at the higher rate: the rate trap.
• Confusing the initial rate with the qualifying rate. Lenders qualify ARMs with a fixed period of 5 years or less on a stress-tested rate (for Fannie/Freddie loans, the higher of the fully-indexed rate or the initial rate + 2%); 7- and 10-year ARMs generally qualify at the note rate. On a short ARM you may qualify for a smaller loan than the initial monthly payment suggests.
• Choosing 3/1 or 5/1 when the fixed-period premium for 7/1 or 10/1 is small. Often the longer fixed period costs only 0.125–0.25% more and cuts reset risk a lot.

Mostly-fixed advice

For most middle-class homebuyers planning to stay 5+ years, the 30-year fixed is the right answer. The ARM is the right answer for a smaller group with specific circumstances:

• Military families with anticipated PCS orders
• People relocating for a job they expect to keep <5 years
• Borrowers who can afford the worst-case payment and want the savings
• Sophisticated investors who actively manage refi opportunities

If you’re unsure which group you’re in, you’re probably in the “take the fixed” group.

Related tools

• Mortgage Calculator to model both side-by-side
• Refinance Breakeven: what an eventual refi would look like
• Affordability, since qualifying differs between fixed and ARM
• ARM vs fixed-rate guide for modern cap structures and the refi-later trap
• Glossary for ARM-specific terms (caps, fully-indexed rate, SOFR)