Fixed vs. ARM — the Honest Comparison

Most mortgage advice about adjustable-rate mortgages comes in two flavors: "ARMs are dangerous, always go fixed" and "ARMs are the smart-money secret." Both are marketing. The honest comparison is quieter: it's a question about your timeline, answered with arithmetic.

What you're actually buying with a 30-year fixed

A 30-year fixed rate is an insurance policy bundled into a loan. The lender promises your rate can never change for three decades, no matter what inflation, markets, or the Fed do. That promise has real value — and therefore a real price. Fixed rates generally sit above the starting rate of a comparable ARM. Call the difference the certainty premium: what you pay for never having to think about what moves rates again.

For many clients that premium is worth every penny. A forever home, a fixed retirement income, a low tolerance for financial suspense — all excellent reasons to buy certainty. Peace of mind is a legitimate line item.

But notice what you're insuring against: rate risk in years you actually hold the loan. Which raises the interesting question.

What an ARM actually is

A modern ARM is fixed first, adjustable later. A "5/6 ARM" is fixed for five years, then adjusts every six months; a "7/6" is fixed for seven years; a "10/6" for ten. During the fixed period it behaves exactly like a fixed-rate loan — same stability, typically at a lower starting rate, because you're not paying for decades of certainty you may never use.

After the fixed period, the rate floats according to a formula written into your note. In plain English:

• Index: a published market rate the loan follows — today most ARMs use SOFR, a broad measure of overnight borrowing costs.
• Margin: a fixed amount added to the index, set at closing and never changing. Index + margin = your new rate at each adjustment.
• Caps: the guardrails, usually quoted as three numbers. The initial cap limits the very first adjustment; the periodic cap limits each adjustment after that; the lifetime cap sets a ceiling the rate can never exceed, period. Before choosing any ARM, you should see the worst-case payment at the lifetime cap in writing — that number, not the teaser, is the loan's true downside. We show it as a matter of course.

No mystery, no fine-print tricks — just a formula you can read before you sign. (Unfamiliar terms live in the glossary.)

The horizon question

Here's the entire decision, in one question: will you still hold this loan when the fixed period ends?

Think about how long people actually keep mortgages. Relocations, upsizing, downsizing, refinancing — most loans end well before their thirtieth birthday. If your realistic horizon is five years — a relocation already sketched, a second home you'll trade up from, an investment property with an exit plan — then a 7/6 or 10/6 ARM gives you fixed-rate certainty across your entire realistic ownership, at a lower rate, and the adjustable years exist only on paper.

In that scenario, the 30-year fixed buyer paid the certainty premium for twenty-five years of insurance they never used.

Flip it around: if you genuinely may stay forever — and plenty of people who "planned to move in five years" are still there in year fifteen — the fixed loan's premium is buying something real. Honesty with yourself about the horizon is the whole game. When your horizon is genuinely unknowable, the fixed rate is the humbler choice, and humility about the future is usually good financial policy.

The refinance fallacy

The most dangerous sentence in this conversation is: "I'll just take the ARM and refinance before it adjusts."

Understand what that sentence assumes: that when your fixed period ends, rates will be attractive enough to refinance into. Nobody knows that — not us, not the Fed, not the people trading bonds for a living. "I'll refi later" is a rate bet, and refinancing also assumes you will qualify later: income, credit, and home value all cooperating on schedule. Any of them can wander over five years.

The clean way to choose an ARM is to make the bet unnecessary: pick a fixed period that covers your realistic horizon on its own. If the plan only works when a future refinance rescues it, the plan is the problem. If the ARM still makes sense assuming you never refinance — you ride to the cap in the worst case and the early savings still justify it — then it's a sound structure, not a gamble.

The same fallacy runs the other direction, incidentally: taking a fixed rate you're unhappy with while "planning to refi the moment rates drop" is the identical bet in a different costume. That impulse is also worth examining before paying points on a loan you don't intend to keep.

How to actually compare

1. Write down your realistic horizon — not the fantasy version.
2. Price the 30-year fixed and the ARMs whose fixed periods cover that horizon.
3. Compute the savings during the fixed period, and the worst-case payment at the caps after it.
4. Ask: does the ARM win even if I never refinance? If yes, it's a candidate. If it only wins with a rescue, take the fixed.

One warning: don't compare these loans by APR — an ARM's APR is built on assumptions about future index values that are guesses wearing decimal points.

The rate widget and Best Option comparison on our home page put fixed and ARM structures side by side on live pricing. And getting pre-approved is where the conversation gets specific — your horizon, your numbers, your call. We've watched every rate cycle since 1997; we'll tell you what the arithmetic says, and the decision stays yours.