What the ARM discount is actually buying
A 7/6 ARM quoted half a point below the 30-year fixed isn't generosity — it's a trade. The lender gives up income in the first seven years in exchange for handing you the interest-rate risk afterward. The fixed-rate borrower pays a premium for certainty they may never use: the median mortgage lasts roughly 7-10 years before a sale or refinance ends it. That's the whole decision in one sentence — if your realistic horizon fits inside the ARM's fixed period, the discount is nearly free money; if you might stay past it, you're short volatility and should know the worst case before signing.
Reading an ARM quote: 7/6, margins, and 2/2/5
- 7/6 ARM = rate fixed for 7 years, then adjusts every 6 months. Modern ARMs adjust off SOFR plus a margin (typically 2.5-3%): index moves, your rate follows.
- Caps come as three numbers, e.g. 2/2/5: the first adjustment can't move more than 2 points, each later adjustment 2 points (per year), and the lifetime cap is 5 points above the start rate. A 5.9% ARM can therefore legally reach 10.9% — that's the number to stress-test, and the worst-case row in this calculator walks that exact path.
- Today's ARMs are not 2008's. The toxic features — negative amortization, interest-only teasers, prepayment penalties, qualification at the teaser rate — were regulated away. Lenders now underwrite you at the fully-indexed rate. The product is honest; the risk is just visible now instead of hidden.
How to think about the horizon input
It's the input that decides everything, so pressure-test it. "We'll move in five years" is a plan, not a fact — job changes fall through, school districts grow on you, and 2021-2023 taught everyone that refinancing out of an ARM assumes rates will cooperate. A useful discipline: take the ARM only if both futures are acceptable — the one where you sell on schedule and pocket the savings, and the one where you're still in the house at the cap-out payment. If the second future breaks the budget, the fixed premium is cheap insurance. Run the payment you'd face against your income with the home affordability calculator.
Banking the difference (the version that actually wins)
The ARM saves money in two channels during the intro period: the lower payment, and faster principal paydown (same payment arithmetic, lower rate — more of each dollar hits principal). The disciplined play is to keep paying the fixed-loan payment on the ARM: the extra $150-200 a month goes straight to principal, shrinking the balance the adjustment will eventually apply to. Done consistently over a 7-year intro period, this cuts thousands off the worst case — the extra payment calculator shows the mechanics. The undisciplined play — absorbing the lower payment into lifestyle — leaves you with the full balance and the full rate risk.
Where each loan wins
- ARM favors: short expected tenure (starter home, likely relocation), high current rate environments where the discount is fat and future refinancing is plausible, borrowers with the income cushion to absorb the cap-out payment, and jumbo borrowers (ARM discounts run larger above conforming limits).
- Fixed favors: the forever home, tight budgets where a $700 payment jump is unaffordable rather than unpleasant, low-rate environments (locking cheap money for 30 years is a gift), and anyone who values never thinking about SOFR again.
- Either way, compare the whole offer, not just the rate — points and fees shift the math; the points calculator and loan comparison calculator handle those trades. And if you're weighing loan length rather than rate type, that's the 15 vs 30 year question.