ARM vs fixed-rate mortgage: when an adjustable rate makes sense

An adjustable-rate mortgage is a bet on your own timeline: a cheaper rate now in exchange for uncertainty later. Modern ARMs are far safer than the 2006 kind, but the bet is still yours to lose.

How a modern ARM works

A “7/6 ARM” has a fixed rate for seven years, then adjusts every six months. The new rate equals an index (today, usually the 30-day average SOFR) plus a fixed margin (commonly 2.5 to 3 percentage points), subject to caps. A typical cap structure is 5/1/5: the first adjustment can move at most 5 points, each later adjustment at most 1 point, and the rate can never exceed the start rate plus 5 points. The loan re-amortizes at each adjustment over the remaining term.

The worst case, in numbers

A $400,000 7/6 ARM starting at 6.0% has a payment of about $2,398. If rates spike, the first adjustment could take it to 11.0% — a payment near $3,700 on the remaining balance. That is the number you must be able to survive or refinance away from; the start rate is not the loan.

When an ARM makes sense

You are confident you will sell or refinance before the first adjustment — a relocation, a planned move-up, a bridge to a known windfall.
The spread between ARM and fixed rates is wide (it is sometimes near zero, which makes the ARM pointless).
You could absorb the capped worst-case payment if your plan slips.
You are buying a jumbo, where ARM discounts are often larger.

When it does not

If this is a long-term home, if your budget is tight at the start rate, or if you are counting on being able to refinance — refinancing requires equity, credit and income at that future moment, none of which are guaranteed. In 2022–2023 many ARM borrowers who planned to refinance found rates two to three points higher than their start rate.

Reading the disclosure

The Loan Estimate for an ARM shows the index, margin, caps, first adjustment date and the maximum possible payment. The federal CHARM booklet from the CFPB walks through the terms. Ask the lender for the payment at the first-adjustment cap and at the lifetime cap in writing.

Frequently asked questions

Can an ARM rate go down?

Yes, if the index falls below the level that produced your start rate (minus the margin). Some ARMs also have a floor rate. Downward adjustments are capped the same way as upward ones.

Are ARMs interest-only?

Most are fully amortizing. Interest-only ARMs exist in the jumbo market and carry a payment shock at the end of the interest-only period in addition to the rate reset.

Can I convert an ARM to a fixed rate?

Only by refinancing, unless your loan includes a rarely offered conversion option. A refinance costs closing costs and requires qualifying again.