ARM vs fixed: which mortgage should you choose?
Last updated September 1, 2026
Take the fixed rate unless you have a specific reason to do otherwise. A fixed loan locks one payment for 30 years and you can stop thinking about it. An ARM starts cheaper, stays fixed for 5, 7, or 10 years, then adjusts with the market twice a year. It wins only when the starting discount is real and you are confident you will be out of the loan before the adjustments start. This page shows you how to test both conditions.
The ARM vs fixed question is really a question about your next decade. Get honest about that first, then the loan choice mostly makes itself.
How does a fixed rate mortgage work?
One rate, set the day you lock, for the whole loan. Your principal and interest payment in month 360 is the same as in month one. Property taxes and insurance still move, but the loan itself never does. That stability is the product. You pay for it with a rate that usually sits above the ARM's starting rate, and you get a payment that no market event can touch.
For a buyer who plans to stay put, fixed is the boring, correct answer. If you are weighing a purchase against staying in your rental, the fixed payment is also what makes the long term math checkable. Run it against your rent in rent vs buy in Los Angeles.
How does a modern ARM work?
Today's ARM is a different animal from the 2006 version. The common conforming products are named like fractions: a 5/6 ARM, a 7/6 ARM, a 10/6 ARM.
How many years the starting rate holds. Five, seven, or ten. During this period the loan behaves exactly like a fixed loan.
How often the rate adjusts after that. Six means every six months. Older products adjusted once a year, which is where names like 5/1 came from.
At each adjustment the new rate is a market index plus a fixed margin written in your note. Conforming ARMs today use an index based on SOFR, the Secured Overnight Financing Rate, which replaced LIBOR. The margin never changes. The index does.
Three caps limit how fast and how far the rate can climb. A floor limits how far it can fall. These numbers are in your note and your Loan Estimate.
Two safety features are standard now and worth naming. The rate is set by a published formula, index plus margin, so the lender does not choose your new rate. And the caps make the worst case computable on day one. You can know, before you sign, the highest payment this loan could ever demand. Compute it. The next section shows how.
What do ARM caps like 2/1/5 mean?
Caps are quoted as three numbers in order. First adjustment, each later adjustment, lifetime. A 2/1/5 cap structure on a loan that starts at 6 percent works like this, and every number here is illustrative.
So in the ugliest market, this loan reaches 8 percent at the first reset, then climbs a point at a time until it hits the 11 percent ceiling, and it can never pass it. The rate can also fall at any adjustment if the index falls, down to the floor.
Cap structures vary. Some products use 5/2/5, where the first adjustment alone can jump 5 points. That is a very different worst case at year one of adjustment. Do not assume. Read the three numbers on your own Loan Estimate and ask the lender to walk you through the worst year they imply.
When does an ARM genuinely win?
When both of these hold at the same time.
First, the discount is real. Compare the ARM and the 30 year fixed quoted to you on the same day. The gap between them moves with the market, and some weeks it is too thin to bother with. An ARM with a starting rate near the fixed rate is risk with no reward.
Second, your horizon is short and you believe it. You are relocating in a known window. This is a bridge house before a planned move up. You are a few years from a retirement downsize. In those cases you may sell before the first adjustment ever arrives, and the fixed loan's extra rate was insurance you never needed.
Here is the size of the prize, with illustrative arithmetic. Take an $800,000 loan. Suppose the 30 year fixed is quoted at 6.5 percent and a 7/6 ARM at 5.75 percent that same day. The fixed payment is about $5,057. The ARM payment is about $4,669. That is roughly $388 a month, and about $32,600 over the seven fixed years. If you sell in year six, you keep all of it and the adjustment never touches you.
Those seven years have to be real, though. The average plan changes. Kids arrive, jobs move, and the house you meant to leave in five years becomes the house you love. Price the ARM as if you might stay, because you might.
What is the honest risk of an ARM?
The payment can rise to the cap, and nobody can promise you it will not. Continue the example. After seven years of payments on that $800,000 ARM at 5.75 percent, about $713,900 of balance remains. Now the adjustments start, and the payment is recalculated on the remaining balance and term at each new rate.
If the first adjustment goes to its 2 point cap, the rate is 7.75 percent and the payment becomes about $5,549. If the market stays bad and the loan grinds up to a 10.75 percent lifetime ceiling, the payment reaches about $6,992. That is roughly $2,300 a month above where you started. The caps did their job, the worst case was computable, and it is still a payment you must be able to survive.
The classic mistake is answering that risk with "I will just refinance before it adjusts." Treat that sentence as a red flag in your own mouth. Refinancing needs three things to line up: rates worth refinancing into, a home value that supports the new loan, and a personal file that still qualifies. In 2021 people assumed all three were permanent. Ask anyone who meant to refinance in 2023 how that went. If your plan only works when a refinance shows up on schedule, your plan is a hope.
One more mechanical note. On conforming loans with shorter fixed periods, the lender may qualify you at a rate above the starting rate, built from the caps or the fully indexed rate. That blunts the "ARM lets me buy more house" pitch, on purpose. Ask your lender what rate they will qualify you at before you fall in love with the ARM payment.
Product mechanics here follow the structure of conforming SOFR indexed ARMs as described in the Fannie Mae Selling Guide and the CFPB's consumer materials on adjustable rate mortgages. Cap structures, margins, floors, and qualifying rules vary by lender and program, and rate spreads change daily. Your own numbers live on your Loan Estimate. Read them there.
So which should you choose?
Use these rules and be strict with yourself.
- Staying put, or unsure? Take the fixed. Uncertainty is a long horizon in disguise, and the fixed loan is the one that forgives a changed plan.
- Short horizon you would bet on, and a real discount? The ARM earns its keep. Bank the monthly savings rather than spending them, and the adjustment risk shrinks further.
- Tempted by the ARM only because the fixed payment does not fit? That is a budget problem, and a loan product cannot fix a budget problem. Revisit the price range and your down payment plan instead.
- Either way, price the worst case. Before you sign an ARM, have the lender show you the fully capped payment in dollars. If that number would break you, the discount is not worth it.
Whichever you pick, compare the actual payments side by side, with taxes and insurance included, in the payments calculator. Loan choice is arithmetic plus honesty about your own timeline. The calculator handles the arithmetic. The honesty is on you.
Four free tools, no sign-up. See what you can afford with Purchase Power. Compare loan scenarios against your rent in the payments calculator. Get every dollar it takes to close in the buyer net sheet. When you are ready to look, get listing matches sent as they hit the market.
This is not lending advice, and every figure on this page is illustrative arithmetic, not a quote. Rates, spreads, cap structures, and qualifying rules change by lender, program, and day. Your own terms come from your lender's Loan Estimate and your note. Have a lender, and if you want a second set of eyes, a fee-only advisor, review them with you.
What is the difference between an ARM and a fixed rate mortgage?
A fixed rate mortgage keeps one rate and one principal and interest payment for the whole loan. An ARM, an adjustable rate mortgage, starts with a fixed rate for a set period, commonly 5, 7, or 10 years, and then the rate adjusts on a schedule, usually every six months, based on a market index plus a fixed margin. Caps limit how far each adjustment can move and how high the rate can ever go.
What does a 5/6 ARM with 2/1/5 caps mean?
The 5 means the rate is fixed for the first five years. The 6 means it adjusts every six months after that. The caps read in order: the first adjustment can move the rate at most 2 points, each later adjustment at most 1 point, and the rate can never rise more than 5 points above where it started. Cap structures vary by lender and program, so read your own loan's numbers on the Loan Estimate.
When is an ARM better than a fixed rate mortgage?
When two things are true at once: the ARM's starting rate is meaningfully below the fixed rate you are quoted the same day, and you are confident you will sell or pay the loan off within the fixed period. If either leg is shaky, take the fixed rate. A plan to refinance before the adjustment does not count as a short horizon, because rates do not owe you a refinance.
Can an ARM payment go down?
Yes. At each adjustment the rate resets to the index plus the margin, within the caps, and the index can fall as well as rise. Most ARMs also have a floor, often the margin itself, below which the rate cannot drop. Plan around the cap, and treat any downward adjustment as a bonus you never counted on.
Summary points
- Fixed is the default. One rate, one principal and interest payment, for the whole loan, and a changed life plan never punishes you for it.
- A modern ARM like a 5/6 or 7/6 holds its starting rate for 5, 7, or 10 years, then adjusts every six months to a SOFR based index plus a fixed margin.
- Caps read in order: first adjustment, each later adjustment, lifetime. A 2/1/5 loan starting at an illustrative 6 percent can never pass 11 percent.
- An ARM wins only when the starting discount is real and you are confident you will be out of the loan before the fixed period ends.
- On an illustrative $800,000 loan, a 0.75 point ARM discount saves about $388 a month, roughly $32,600 over a seven year fixed period.
- The honest risk: the same illustrative loan fully capped reaches a payment about $2,300 a month above where it started. Price the capped payment in dollars before you sign.
- Never buy an ARM on the assumption you will refinance before it adjusts. Rates do not owe you a refinance.