Fixed vs. Adjustable-Rate Mortgage: Which Should You Choose?

A fixed-rate mortgage locks your rate for the life of the loan. An adjustable-rate mortgage starts lower and then moves with the market. The choice is usually described as certainty versus a cheaper start, which is true but too vague to decide anything — so this guide puts both ends of the range in dollars, because the worst case is knowable in advance and almost nobody calculates it.

How to read an ARM

ARMs are quoted as two numbers, like 5/1 or 7/6. The first is how many years the rate stays fixed; the second is how often it adjusts afterwards — annually for a 5/1, every six months for a 7/6. After the fixed period the rate becomes an index that tracks market rates, plus a fixed margin set in your contract.

The margin is the part to negotiate and the part nobody reads. The index moves for reasons outside anyone's control, but the margin is yours for the life of the loan, and a difference of half a point between lenders costs the same as half a point of rate.

The caps define your worst case

Every ARM limits how far the rate can move, usually written as three numbers such as 2/2/5: the maximum increase at the first adjustment, at each later one, and over the life of the loan. Those caps convert an open-ended risk into a specific number, and that number is what you should be deciding against.

Take a $350,000 loan: a 5/1 ARM at 5.75% against a 30-year fixed at 6.75%, with 2/2/5 caps.

• ARM payment — $2,043 a month. Fixed payment — $2,270. The ARM saves $228 a month, about $13,700 across the five fixed years.
• At the first adjustment, capped at +2%, the rate becomes 7.75% and the payment $2,452 — $410 a month more than it started.
• At the lifetime cap of 10.75%, the payment reaches $3,124 — $1,081 a month above the starting payment, and $854 above the fixed loan you declined.

That is the trade stated honestly: $13,700 of certain saving against a possible $1,081 a month. The question is not whether rates will rise, which nobody knows. It is whether you could pay $3,124 if they did.

When an ARM is reasonable

• You will genuinely be gone before the fixed period ends — a role with a known relocation, military orders, a house you have already decided is temporary.
• Rates are high and expected to fall, making the fixed alternative expensive to lock in. An ARM can be refinanced later, though this relies on being able to qualify then.
• The worst-case payment is affordable. If you could carry $3,124 without distress, the ARM is a calculated saving rather than a gamble.
• You are borrowing well below what you qualify for, so the cap has room to bite without threatening anything.

The common thread is that every good reason survives the worst case. A reason that only works if rates stay low is not a reason, it is a forecast.

Where the plan usually breaks

“I will refinance before it adjusts” is the most common ARM plan and the most fragile one. Refinancing requires qualifying again, which depends on your income, your credit and the home's value at that moment — and the conditions that push rates up are the same conditions that soften house prices and job security. The option is real, but it is least available exactly when you would need it most.

Selling has the same weakness in miniature. It works, unless the market you must sell into is the one that also raised your rate.

The 2008 association, and what changed

ARMs carry a reputation from the financial crisis that is worth separating from today's product. The loans that caused the damage were largely a different animal: option ARMs and negative-amortisation products, frequently issued without documented income, where the balance could grow while the borrower made the minimum payment.

Those structures are effectively gone. A modern ARM is fully underwritten, amortising, and — under the ability-to-repay rules — generally assessed against a higher rate than its teaser. The remaining risk is straightforward and disclosed rather than hidden. That is a real improvement, and it is not the same as the loan being safe for everyone.

If you already hold one that is about to adjust

Find three things in your note before doing anything: the exact first adjustment date, the index and margin, and the three caps. Together they tell you the worst payment you could face at the next adjustment, which is usually less alarming than the lifetime cap and occasionally worse than expected.

Then decide with time to spare. Refinancing takes weeks, and doing it before the adjustment rather than after it avoids paying the higher rate in the interim. If refinancing is not available — insufficient equity, changed income — the fallback is to reduce the balance before the recalculation, since the new payment is computed on what you owe at that moment.

When fixed is the right answer

For most buyers, most of the time. A fixed rate makes the largest expense in your life a constant, which is worth a great deal when everything else about a decade is unknown. It also has a genuine asymmetry in its favour: if rates fall you can refinance into the gain, while an ARM holder absorbs a rise they cannot refuse.

The teaser saving is also smaller than it looks once you account for that asymmetry. Paying $228 a month for the right to be certain, on a debt you will hold for decades, is not obviously bad value.

Run your own number

Compare a fixed loan against an ARM's starting rate in the ARM vs fixed calculator — then run the mortgage calculator again at the ARM's lifetime cap and ask whether that payment is survivable, which is the only version of this decision that matters. Check the result against your budget in the home affordability calculator, and price the exit in the refinance calculator before relying on it.