ARM vs. Fixed-Rate Mortgages
A fixed rate never moves for the life of the loan. An adjustable-rate mortgage (ARM) starts lower, then can move — up or down — on a set schedule. The real question isn't which is "safer." It's how long you'll actually keep the loan.
How an ARM is built
ARMs are quoted like 7/6 or 5/6: the first number is how many years the intro rate is fixed; the second is how often it adjusts afterward (every 6 months). Once the intro period ends, your rate is rebuilt from two parts — a public index plus a fixed margin the lender set at closing:
- The index is a market benchmark, almost always SOFR (the Secured Overnight Financing Rate) on modern loans. It moves with the market — this is the part you don't control.
- The margin is a fixed spread the lender adds on top, typically around 2.75% on conforming loans (Freddie Mac's guidelines hold it between 1% and 3%). It's set at closing and never changes for the life of the loan.
So at each adjustment: new rate = current SOFR + your margin, then bounded by the caps below. The margin is also your floor — if the index fell to zero, your rate still couldn't drop below the margin. Ask for the index and margin in writing before you sign; they define every future payment.
The caps that contain it
Every ARM carries rate caps, usually written as three numbers like 2/2/5: how much the rate can jump at the first adjustment (+2%), at each later adjustment (+2%), and over the life of the loan (+5% above the start rate). Read the lifetime cap as your worst case, not a footnote. If you can't comfortably afford the payment at that ceiling, the intro rate is a mirage — always qualify yourself at the lifetime cap, not the teaser.
A worked example
Take a $400,000 loan and compare a 7/6 ARM against a 30-year fixed, at illustrative recent rates. The ARM's lower intro rate saves about $260 a month for the first seven years — roughly $21,800 if you're gone before it adjusts. But if it climbs to its lifetime cap afterward, the payment on the remaining balance jumps well past the fixed loan you passed on:
| Scenario | Rate | Monthly P&I |
|---|---|---|
| 7/6 ARM — intro (years 1–7) | 5.75% | $2,334 |
| 30-year fixed | 6.75% | $2,594 |
| Same ARM at its 2/2/5 lifetime cap (year 8+) | 10.75% | ≈ $3,500 |
Illustrative P&I only, at sample rates; your numbers depend on the day's market and your file. Run yours on the calculator.
When an ARM is rational
The ARM's lower intro rate is compensation for taking on that future risk. It pays off when your horizon is shorter than the fixed period: a known relocation, a starter home you'll sell in a few years, or a loan you're confident you'll pay off early. In those cases you capture the lower rate and are gone before the adjustment ever bites.
When fixed wins
If you'll hold the loan long-term, a fixed rate is certainty you can build a budget around — and "we'll just refinance later" quietly assumes rates will be lower and that you'll still qualify. Neither is guaranteed. Because fixed mortgage rates track the bond market (here's how), you can shop today's fixed rate against our neutral rate board and know exactly what you're locking in.
Common questions
What does a 7/6 ARM mean?
The first number is how many years the intro rate stays fixed (7); the second is how often it can adjust after that (every 6 months). A 5/6 ARM fixes the rate for 5 years, then adjusts every 6 months.
What happens when my ARM adjusts?
The lender rebuilds your rate as the current index (usually SOFR) plus your fixed margin, then applies the caps. That new rate is applied to your remaining balance over the remaining term, so the payment is recalculated at each adjustment.
Can my ARM payment ever go down?
Yes — if the index falls, your rate (and payment) can drop at an adjustment. But it can never fall below your margin, which acts as a floor, and per-adjustment caps limit how far it moves in either direction.
What are typical ARM caps?
A common structure is 2/2/5: up to +2% at the first adjustment, +2% at each later one, and +5% over the life of the loan. Always check you could afford the payment at that lifetime cap before choosing an ARM.
Is an ARM riskier than a fixed-rate loan?
It shifts interest-rate risk to you once the intro period ends. That is rational when your horizon is clearly shorter than the fixed period; it gets riskier the longer you might hold the loan, because 'we'll just refinance' assumes both lower rates and that you'll still qualify.
By L.W. Martin, Founder — 20 years in the mortgage business, including 15 running his own brokerage. About → · Updated September 2026.