Part of our guide to How Much House Can You Actually Afford?
The useful way to think about an adjustable-rate mortgage is not "a loan whose rate changes". It is a fixed-rate loan with an expiry date, and the whole decision is about what happens when it expires.
An ARM gives you a lower rate for a fixed opening period, then adjusts to an index plus a fixed margin for the rest of the term. Three caps limit how far it can move. So the question is simply: will you still own this loan when the fixed period ends? If you are confident you will not, the ARM's saving is real. If you are relying on being able to refinance, you are relying on conditions nobody can promise.
Reading the numbers
An ARM is written as two numbers, such as 5/1 or 7/6.
- The first is the years the initial rate is fixed
- The second is how often it adjusts afterwards —
1for once a year,6for every six months
So 7/6 means seven years fixed, then adjusting twice yearly for the remaining term.
How the new rate is set
Two components, and only one of them moves.
The index tracks market rates. Most new US ARMs use SOFR, which replaced LIBOR. It rises and falls with the wider market and nobody controls it.
The margin is a fixed number added to the index, set in your contract at the start. It never changes for the life of the loan.
new rate = index + marginBecause the margin is fixed and disclosed upfront, it is worth comparing between lenders. Two ARMs with identical teaser rates can carry different margins, and that difference persists for decades after the introductory period ends.
The three caps
These define your worst case, and they are the most important numbers in the contract.
| Cap | Limits |
|---|---|
| Initial | How much the rate can move at the first adjustment |
| Periodic | How much it can move at each adjustment after that |
| Lifetime | How high it can ever go, total |
Find the lifetime cap, apply it to your loan, and calculate that monthly payment. That is the payment you are agreeing you could afford. If that number is not survivable, the ARM is not suitable regardless of how attractive the opening rate looks.
Doing this takes five minutes and is the single most useful thing a borrower considering an ARM can do.
Most people choosing an ARM plan to refinance or sell before it adjusts. That is a reasonable plan and it is not a guarantee.
Refinancing in five or seven years depends on your credit then, your income then, and your home's value then. If values have fallen, or your circumstances changed, or rates are higher, the refinance you were counting on may not be available on terms you like.
The honest test is not "will I probably refinance?" It is "can I afford this loan at its lifetime cap if I cannot?" An ARM you could survive at the cap is a calculated choice. One you could not is a bet on the future.
When each one fits
Fixed suits you if you plan to stay a long time, you value a payment that cannot move, you are stretching to afford the house, or rates are historically low and worth locking.
An ARM suits you if you have a genuine reason to expect to leave before adjustment — a known relocation, a defined career move — or the gap between the ARM and fixed rate is large enough that the early saving matters and you could absorb the cap.
Neither is intrinsically better. An ARM is not a trap and a fixed rate is not always prudent — paying a premium for certainty you do not need is also a cost.
Two details worth checking
Prepayment penalties. Less common now, but confirm there is none, particularly if the plan involves refinancing.
Conversion options. Some ARMs allow conversion to a fixed rate under set terms. Where offered it is worth understanding upfront rather than discovering later.
What to do before signing
- Find the margin, and compare it between lenders — not just the teaser rate
- Find all three caps
- Calculate the payment at the lifetime cap, and decide honestly whether you could pay it
- Ask what index the loan uses
- Confirm no prepayment penalty
- Compare against a fixed quote for the same house, over the years you actually expect to stay
If the capped payment is comfortable and you have a real reason to expect an early exit, the ARM is a rational choice. Otherwise the fixed rate is buying you something worth having.
For the wider picture, see how much house you can afford and what closing costs actually pay for.
This is general information, not financial advice — see our disclaimer.
Frequently asked questions
What does 5/1 or 7/6 mean?
The first number is how many years the rate stays fixed. The second is how often it adjusts afterwards — 1 for annually, 6 for every six months. So 7/6 means fixed seven years, then adjusting twice a year.
How is the new rate calculated?
An index that moves with the market, plus a fixed margin set in your contract. The margin never changes; the index does. Most new US ARMs use SOFR as the index.
What are the caps?
Three limits on how far the rate can move — on the first adjustment, on each subsequent one, and over the life of the loan. They are in your loan documents and they define your worst case.
Is an ARM ever the better choice?
Yes, when you are confident you will sell or refinance before the fixed period ends, or when the starting rate is far enough below fixed that the early saving is worth the later uncertainty.
What is the main risk?
Assuming you will refinance before adjustment. Refinancing depends on your credit, your income and your home's value at that future moment — none of which is guaranteed.
Sources
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