Adjustable-Rate Mortgage (ARM) Explained: How It Works, the Caps and the Real Risk

An adjustable-rate mortgage (ARM) starts with a fixed rate for a set number of years, then the rate can go up or down on a schedule. Right now the starting rate on an ARM is lower than a 30-year fixed: in the Mortgage Bankers Association's survey for the week ending Oct. 2, 2026, the average 5/1 ARM contract rate was 6.43%, while the 30-year fixed was 7.49%. The trade-off is simple: you get a lower payment now, and you take on the risk that it rises later.

What is an adjustable-rate mortgage?

The Consumer Financial Protection Bureau (CFPB) puts it plainly: ""With an adjustable-rate mortgage, the interest rate may go up or down."" A fixed-rate loan keeps the same rate for the whole loan. An ARM does not.

Most ARMs today are ""hybrid"" ARMs. The rate stays fixed for the first few years. After that, it resets on a set schedule for the rest of the loan.

The name tells you how it works. The CFPB explains that in a 5/1 ARM, the ""5"" is ""the number of years your initial interest rate will stay fixed"" and the ""1"" is how often the rate adjusts after that. Many ARMs sold today adjust every six months, so you will also see names like 5/6, 7/6 and 10/6.

How does an ARM rate change after the fixed period?

Once the fixed years end, your new rate is built from two parts:

• The index. The CFPB calls it ""a benchmark interest rate that reflects general market conditions."" For the standard ARMs Fannie Mae buys, the index is ""a 30-day average of the SOFR index as published daily by the Federal Reserve Bank of New York.""

• The margin. This is ""the number of percentage points added to the index by the mortgage lender,"" per the CFPB. The margin is set in your loan papers and does not change.

Index plus margin is called the ""fully indexed rate."" When your rate resets, it moves toward that number, but only as far as your caps allow.

What are ARM rate caps?

Caps are the limits that protect you from a huge jump. The CFPB lists three kinds:

• Initial adjustment cap: ""how much the interest rate can increase or decrease the first time it adjusts.""

• Subsequent adjustment cap: ""how much the interest rate can increase or decrease in the adjustment periods that follow.""

• Lifetime adjustment cap: ""how much the interest rate can increase or decrease in total, over the life of the loan.""

Caps are often written as three numbers. On Fannie Mae's Standard ARM Plan Matrix, the 5/6 SOFR ARM has caps of 2/1/5: up to 2 points at the first change, 1 point at each change after that, and no more than 5 points over the loan. The 7/6 and 10/6 SOFR ARMs on the same matrix use 5/1/5.

FHA ARMs follow HUD's own rules. HUD says 1- and 3-year FHA ARMs can rise one point a year and five points over the life of the loan. Five-year FHA ARMs use either that same 1/5 setup or 2 points a year and 6 over the life. Seven- and 10-year FHA ARMs can rise 2 points a year and 6 over the life.

How much can an ARM save, and how high can the payment go?

Here is a worked EXAMPLE on a $400,000 loan. It uses MBA's average contract rates for the week ending Oct. 2, 2026 (5/1 ARM 6.43%, 30-year fixed 7.49%) and the 2/1/5 caps described above. Payments are principal and interest only. Taxes and insurance are extra.

In this example the ARM costs about $284 less a month for five years. That adds up to about $17,050 before the first reset. After 60 payments, about $374,100 of the loan is still owed, and the year-6 payments above are figured on that balance over the 25 years left.

The worst case is the number to plan around. If you could not handle about $3,784 a month, think hard before choosing an ARM on that loan size. This is an example only. Your lender's real rate, margin and caps are on your Loan Estimate.

Can an ARM help you qualify for a bigger loan?

Often not as much as buyers expect. Fannie Mae's Selling Guide says lenders must ""limit the impact of any potential payment shock on an ARM with an initial fixed-rate period of five years or less"" by qualifying you at a special rate.

For a 5-year SOFR ARM with 2/1/5 caps run through Fannie Mae's Desktop Underwriter, that qualifying rate is the ""greater of fully indexed rate or note rate plus 2%."" So if your ARM starts at 6.43%, the lender may test your budget at 8.43% or higher, not at 6.43%. For 7- and 10-year SOFR ARMs, the same table says the qualifying rate is the note rate.

In plain words: a 5-year ARM can lower your first payment, but it may not raise the price you qualify for. A 7- or 10-year ARM is treated differently. Ask your lender to show you both.

How popular are ARMs right now?

Still a small slice of the market. MBA reported that the ARM share of applications ""remained unchanged at 10.3% of total applications"" in the week ending Oct. 2, 2026. The other nine in ten applications were for fixed-rate loans.

For context, Freddie Mac's weekly survey had the 30-year fixed at 7.40% as of Oct. 8, 2026, up from 6.30% a year earlier. The 15-year fixed was 6.73%. When fixed rates climb, more buyers start asking about ARMs.

Is an ARM a good idea for you?

An ARM can make sense if most of these are true:

• You expect to sell or refinance before the fixed period ends, and you have a backup plan if you can't.

• You could still afford the payment at the lifetime cap.

• You have savings to cover a higher payment for a while.

• You picked a fixed period long enough to match your plans. A 7- or 10-year ARM gives more breathing room than a 5-year one.

A fixed rate is usually the better fit if you plan to stay a long time, your budget is tight, or a rising payment would keep you up at night. Nobody can promise where rates will be in five or seven years, and refinancing is never guaranteed.

What this means for new-construction buyers

New construction adds a timing twist. A home being built may not close for months, and your rate is not set until you lock. Read our guide to when to lock your mortgage rate and how long it takes to build a house before you choose a loan type.

Builders also offer rate tools. A temporary buydown, like a 2-1 buydown, lowers the rate for the first year or two on a fixed loan. A permanent buydown lowers it for good. Compare those offers side by side with an ARM using our guide to permanent vs temporary buydowns. A fixed loan with a buydown has a payment that never goes above the full note rate. An ARM's payment can.

Three questions to ask before you sign:

1. What is the index, the margin and the cap structure on this ARM?

2. What payment would I owe at the first reset and at the lifetime cap?

3. Can I lock the rate long enough to cover the build, and what does a longer lock cost?

Common Questions

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Call 561-704-0091 to talk with a National House Search specialist, or Schedule a visit.

Related reading: When to lock your mortgage rate · 2-1 buydown explained · Permanent vs temporary buydowns · How long does it take to build a house?

Sources: CFPB: fixed vs adjustable-rate mortgages · CFPB: ARM rate caps · Fannie Mae Selling Guide B3-6-04 · HUD: FHA ARMs · Freddie Mac PMMS

This is general information, not financial, tax or legal advice. Figures checked October 9, 2026."