Adjustable-rate mortgages earned their bad reputation honestly, in 2008, when option ARMs with hidden negative amortization blew up on borrowers who never understood them. Today's ARMs are different animals — heavily regulated, with defined limits — but the core discipline is the same: the introductory rate is the least important number on the page. What matters is the machinery that takes over when it expires.
The anatomy of a modern ARM
A modern conforming ARM is written as two numbers, like 7/6. The first is how many years the rate is fixed — seven. The second is how often it adjusts after that, in months — every six. So a 7/6 ARM is fixed for seven years, then re-prices twice a year for the remaining twenty-three.
The common flavours are 5/6, 7/6 and 10/6. If you remember the old 5/1 and 7/1 ARMs that adjusted annually, those are largely gone from the conforming market. When the industry retired LIBOR in 2023 and moved to the SOFR index, it also switched from annual to six-month adjustments — and, to compensate, tightened the per-adjustment cap. More on that below.
SOFR and the margin: where your future rate comes from
Once the fixed period ends, your rate is built from two pieces added together:
The index is the Secured Overnight Financing Rate, which the New York Federal Reserve publishes daily based on actual overnight lending against U.S. Treasuries. It rises and falls with the market. The margin is your lender's fixed markup, set on the day you close and never changed thereafter — capped at 3% on conforming loans, and most commonly 2.75%. Add them and you get the fully-indexed rate: what you would actually pay at the next adjustment, before the caps are applied.
Two consequences worth internalising. First, the margin is the part you can shop — two lenders quoting the same teaser can carry different margins, and the margin is forever, so compare it. Second, there is a floor: your rate can never drop below the margin during the adjustable period. If SOFR went to zero, a 2.75% margin means a 2.75% rate, no lower.
The three caps, which are the actual loan
Between you and an unbounded rate stand three caps, written as three numbers like 5/1/5. This is the most important line in the entire document, and it is the one borrowers skip.
Notice the trade-off baked in. The 5/6 ARM gives you only five fixed years but a gentle 2% cap on the first adjustment. The 7/6 and 10/6 give you more fixed years but allow a brutal 5% jump the first time they move. Longer safety up front, bigger shock at the cliff.
The periodic cap of 1% is the quiet piece of good news: after the first adjustment, the rate can only creep by one percentage point every six months — two points a year — so even a sustained rate spike takes years to reach the lifetime ceiling.
A 7/6 ARM, best case and worst case
Take the house example: $400,000 over 30 years. Suppose a 7/6 ARM offers an intro rate of 5.75% against a 30-year fixed at 6.50%.
7/6 ARM at 5.75% vs 30-year fixed at 6.50%, $400,000
Two very different futures. In the benign case you banked $16,294 over seven years and your payment ticks up modestly. In the worst case your payment leaps by $1,161 a month — and you need to know, before signing, whether you could survive that.
The worst case is not a forecast; it is a boundary. Rates would have to rise and stay high for it to fully materialise, and the 1% periodic cap means it arrives in steps, not overnight. But the whole point of an ARM is that you are the one carrying that risk, so you must be able to price it.
The rate they qualify you at is not the teaser
Lenders know the teaser is temporary, so federal ability-to-repay rules require them to qualify you at the higher of the fully-indexed rate or your start rate plus 2%. On our example that is the greater of 7.05% or 7.75% — so 7.75%, not 5.75%. You must show you can afford the loan at that higher rate to be approved. This is a feature, not an obstacle: it is the regulation that stops the 2008 mistake from repeating, and if you cannot clear it, the ARM was going to hurt you.
When an ARM is the rational choice
An ARM is a genuinely smart instrument in specific situations:
- You will be gone before the cliff. If you know you will sell or refinance within the fixed period — a job you expect to move for, a starter home you will outgrow — you capture the lower rate and never face an adjustment. Match the fixed period to your horizon with room to spare: take a 7/6, not a 5/6, if you think you will move in year six.
- Fixed rates are high and you expect to refinance down. When the yield curve makes ARM rates meaningfully cheaper than fixed, an ARM lowers your payment now, and you refinance to a fixed loan if rates fall. You are betting you can refinance — a bet that fails if rates rise and your equity or income deteriorate at the same time.
- You expect to pay the loan down fast. A large expected windfall or aggressive prepayment plan shrinks the balance that would be exposed to a higher rate.
When it is a trap
Against all of that, the case for a fixed loan is simple and strong for most people: you are buying certainty, and certainty is worth paying for when the asset is your home.
- You will hold the house long-term. If this is where you intend to stay, a 30- year fixed removes the single largest variable in your budget for three decades. That is what the product is for. The 15-year-versus-30-year decision is its own piece: the real trade-off.
- The spread is thin. If the ARM saves only a fraction of a point over the fixed rate, you are taking on years of interest-rate risk for very little. The saving has to be worth the exposure.
- You are relying on being able to refinance. Refinancing requires qualifying again — income, credit, appraisal — at the future moment. If you are counting on it as your escape hatch, remember the hatch can be bolted shut exactly when you need it, as the liquidity argument explains.
The rule of thumb: take an ARM when you have a concrete reason the adjustment will never touch you, and take a fixed loan when you don't. “Rates might fall” is a hope, not a plan.
Frequently asked questions
What does 7/6 mean on an ARM?
The rate is fixed for 7 years, then adjusts every 6 months for the rest of the term. The first number is the fixed period in years; the second is the adjustment frequency in months. Common versions are 5/6, 7/6 and 10/6. The older annually-adjusting 5/1 format has largely disappeared from the conforming market since the 2023 move to the SOFR index.
What index do ARMs use now that LIBOR is gone?
The Secured Overnight Financing Rate (SOFR), published daily by the New York Federal Reserve. Conforming ARMs use the 30-day average SOFR plus a fixed margin (capped at 3%, usually 2.75%). SOFR replaced LIBOR when the industry retired it in 2023.
What do the three cap numbers mean?
Written as initial/periodic/lifetime — for example 5/1/5. The first is the maximum rate increase at the first adjustment; the second is the maximum at each later adjustment; the third is the maximum increase ever, over the life of the loan. In 2026, 5/6 ARMs are typically 2/1/5 and 7/6 and 10/6 ARMs are 5/1/5.
How high can my ARM payment actually go?
Up to the lifetime cap — typically 5 percentage points above your start rate. On a 5.75% start that is 10.75%, which on a $357,000 balance is roughly $1,161 more per month than the intro payment. Your Loan Estimate and Closing Disclosure state this maximum explicitly, and you should confirm you could afford it before signing.
Is an ARM a good idea in 2026?
Only if you have a concrete reason the adjustment will not reach you — you will sell or refinance within the fixed period, or you will pay the balance down fast. If you plan to hold the home long-term, a fixed rate buys certainty that is usually worth the modest extra cost. 'Rates might fall' is a hope, not a plan.
Mortgage Ledger publishes educational information, not personalized financial, legal, tax, lending, or investment advice. The figures here are estimates built on stated assumptions and will not match a lender’s underwriting exactly. Confirm any number that matters against your Loan Estimate and a licensed professional before you act on it.