Every mortgage answers one question before anything else: who carries the risk that interest rates move? A fixed-rate loan puts it on the lender. An adjustable-rate mortgage puts it on you, and pays you an initial discount for taking it.
Everything else about the two products follows from that single difference.
Fixed-rate: the price of certainty
A fixed-rate mortgage locks the interest rate for the full term. The principal-and-interest portion of your payment is identical in month 1 and month 360. Property taxes and insurance still move, so your total escrowed payment is not truly fixed, but the part the lender controls is.
The lender is now exposed. If market rates rise sharply, they are stuck holding a loan yielding less than they could get today. They charge for that exposure, which is why fixed rates start above comparable ARM rates.
The recent Freddie Mac survey reference point for a 30-year fixed was 6.58% as of late July 2026 — a snapshot, not a live quote, and one that moves weekly.
Adjustable-rate: the mechanics
An ARM fixes the rate for an initial period, then adjusts periodically for the rest of the term. The naming convention tells you both: in a 5/1 ARM, the rate is fixed five years then adjusts annually; in a 7/6 ARM, fixed seven years then adjusts every six months.
After the initial period, your rate is the index plus the margin. The index is a published market rate — most US ARMs now use SOFR, which replaced LIBOR after its retirement. The margin is a fixed number written into your note, typically somewhere in the low single digits, and it never changes.
So if SOFR sits at 3.64% at an adjustment date and your margin is 2.75%, the fully indexed rate is 6.39%, subject to the caps.
The caps are the whole risk profile
Caps limit how far the rate can move, and they are usually written as three numbers such as 2/2/5.
The first is the initial adjustment cap: the most the rate can change at the first adjustment. The second is the periodic cap: the most it can change at each adjustment after that. The third is the lifetime cap: the most it can ever exceed the initial rate.
Take a 5/1 ARM starting at 5.75% with 2/2/5 caps. Year six could be as high as 7.75%. Year seven as high as 9.75%. The lifetime ceiling is 10.75%. On a $400,000 loan, that is the difference between a roughly $2,334 payment and a roughly $3,733 one.
This is the number that matters, and it is knowable in advance. If you could not make the lifetime-cap payment, the ARM is not a rate bet — it is a solvency bet.
When an ARM genuinely makes sense
The clean case is a known short horizon. A household certain they will move within five years — a fixed-term assignment, a training program with a defined end, a job in a field that relocates — can take a 7/1 ARM and never reach an adjustment.
The second case is a borrower who could absorb the lifetime cap without difficulty and wants the initial savings. If the ARM starts 0.75 points below the fixed rate on a $400,000 loan, that is roughly $190 a month for the initial period, or about $11,400 over five years.
The weak case, and the common one, is a borrower who takes the ARM because it is the only payment they qualify for and plans to refinance later. That plan depends on future rates, a future appraisal, and future income all cooperating.
When fixed is the obvious answer
If you intend to stay long-term, if your income is tight against the payment, or if a payment increase would force a sale, the certainty is worth the premium. You are buying the removal of a specific catastrophic outcome, and that is what insurance is for.
A fixed-rate loan also carries a valuable asymmetry: if rates fall meaningfully, you can refinance into the lower rate. If they rise, you keep the old one. The lender bears the downside and you keep the option — which is precisely why the starting rate is higher.
Questions to ask before signing an ARM
What is the index and the margin? What are the three caps? What is the payment at the lifetime cap, and could I make it? Is there a prepayment penalty in the initial period? Is the initial rate a genuine fully indexed rate or a discounted teaser that will jump at the first adjustment even if the index does not move?
That last one catches people. If the initial rate is below index plus margin at origination, your first adjustment goes up regardless of what happens in the market.