Adjustable-rate mortgages, in depth: caps, indexes, and margins

An adjustable-rate mortgage trades certainty for a lower starting rate: you get a fixed rate for an initial period, then the rate floats with the market for the rest of the loan. That trade can be smart or dangerous depending on details most borrowers never examine — the index it follows, the margin added on top, and the caps that limit how far it can move. ARMs aren't inherently bad; they're a specialist tool. Using one well means understanding exactly what your payment can become in the worst case, not just what it starts at.

Reading the name: what '5/6 ARM' means

ARM names encode the schedule. A 5/6 ARM is fixed for the first 5 years, then adjusts every 6 months thereafter. A 7/6 is fixed 7 years, then adjusts semiannually; older '5/1' notation meant annual adjustments. The first number is your fixed runway; the second is how often it re-prices after that. The longer the fixed period, the more the ARM behaves like a fixed loan up front — and the higher its starting rate relative to a shorter ARM.

How the adjusted rate is calculated

When the fixed period ends, your new rate is built from two pieces: an index plus a margin. The index is a published benchmark that moves with the market (modern ARMs commonly use SOFR-based indexes). The margin is a fixed number of percentage points the lender adds, set in your loan documents and unchanging for the life of the loan. Index plus margin equals your 'fully indexed rate.' Because the margin never changes, the index is what makes your payment move — and the margin is negotiable at origination, so it's worth asking about.

The caps that limit the damage

Caps are the guardrails, and they usually come as three numbers, often written like 2/1/5. The first is the initial cap: how much the rate can jump at the very first adjustment. The second is the periodic cap: how much it can change at each subsequent adjustment. The third is the lifetime cap: the maximum the rate can ever rise above your starting rate. Those three numbers define your worst case — and the worst case, not the teaser rate, is what you should qualify yourself against.

When an ARM makes sense — and when it doesn't

• It fits a genuine short horizon: you're confident you'll sell or refinance within the fixed period — a known relocation, a starter home with a real timeline.
• It fits when the ARM discount is large: sometimes the initial rate is meaningfully below fixed rates, and you'll be gone before it adjusts.
• It backfires as a way to afford more house: using the low initial payment to stretch into a home you can't carry at the fully indexed rate is the classic ARM trap.
• It backfires on hope: 'I'll refinance before it resets' assumes rates and your finances cooperate on schedule — the bet that hurt borrowers in 2008.

The bottom line

An ARM is a fixed-then-floating loan whose real character lives in its details: the fixed period, the index and margin that set the adjusted rate, and the initial, periodic, and lifetime caps that bound it. Ask for all of those numbers, compute the fully indexed rate today, and qualify yourself against the lifetime cap. Used with a real exit plan and a survivable worst case, an ARM is a legitimate tool. Used to reach a house you can only afford at the teaser rate, it's a trap wearing a discount.