Adjustable Rate Mortgage: How It Works, Structures, and Caps

An adjustable-rate mortgage, or ARM, is a home loan that starts with a fixed interest rate for a set number of years and then adjusts periodically based on a market index plus a fixed margin set by your lender. The introductory rate is usually lower than what a comparable 30-year fixed loan would offer, which is the trade you’re making: cheaper payments now in exchange for payments that can rise later. Caps written into every ARM limit how far the rate can move at each adjustment and over the life of the loan, so the uncertainty has boundaries you can calculate before you sign.

How the Rate Is Calculated

Every ARM uses the same formula once the fixed period ends: an index plus a margin equals your interest rate. The index is a published benchmark that reflects current borrowing costs in the broader economy and moves on its own. The margin is a fixed percentage the lender adds on top, and it stays the same for the life of the loan. Add them together and you get the fully indexed rate, which is what you actually pay.

If the index sits at 4.0% and your margin is 2.75%, your fully indexed rate is 6.75%. If the index drops to 3.5% at the next adjustment, your new rate is 6.25%. The margin never budges, so the index is the only moving part. Federal regulations require lenders to tell you which index your loan tracks and where you can watch it yourself.¹ Lenders cannot pick an index they themselves control.

Common ARM Structures

Lenders identify ARMs with two numbers separated by a slash. The first is the length of the fixed introductory period in years. The second is how often the rate adjusts after that.²

5/6 ARM: fixed for five years, then adjusts every six months. This is now the most common ARM on the market.
5/1 ARM: fixed for five years, then adjusts once a year.
7/1 ARM: seven years of fixed payments before annual adjustments begin.
10/1 ARM: a full decade at the introductory rate, with annual changes starting in year eleven.

These are often called hybrid mortgages because they combine fixed-rate stability at the start with variable-rate behavior later. The introductory rate is typically lower than what you’d pay on a comparable 30-year fixed loan, which is the main draw for buyers who expect to sell or refinance before the adjustments start.³

Rate Caps That Limit Adjustments

Caps are the guardrails on how far your rate can move. Every ARM has three of them, and the exact numbers are spelled out in your loan documents.

Initial adjustment cap: limits how much the rate can change the first time it adjusts after the fixed period ends, commonly two or five percentage points.⁴
Subsequent adjustment cap: limits each rate change after the first one, typically one or two percentage points.
Lifetime cap: the absolute ceiling for how far the rate can ever rise above the initial rate, most commonly five percentage points. Some loans also set a floor.

You’ll see these written as shorthand like 2/2/5 or 5/2/5. The numbers represent the initial cap, the subsequent cap, and the lifetime cap, in that order. Take a 5/1 ARM starting at 5.0% with a 2/2/5 cap structure: it could jump to 7.0% at the first adjustment, move another two points at the next, and never exceed 10.0% over the loan’s life. Running that worst-case math before you sign tells you whether you can live with the ceiling payment.

Which Index Your Loan Tracks

Two indices dominate the ARM market today, and both are rooted in U.S. Treasury markets.

The Secured Overnight Financing Rate (SOFR) measures the cost of borrowing cash overnight using Treasury securities as collateral. It’s published daily by the Federal Reserve Bank of New York and reflects actual transaction data rather than estimates.⁵ SOFR replaced LIBOR as the standard ARM index after regulators phased out LIBOR at the end of June 2023. Existing LIBOR-indexed ARMs were transitioned to a spread-adjusted version of SOFR designed to produce roughly equivalent rate outcomes for borrowers already in those loans.⁶

The Constant Maturity Treasury (CMT) index comes from the Treasury Department’s daily yield curve, which tracks what investors are earning on government bonds at specific maturities. A 1-year CMT rate represents the yield on a theoretical one-year Treasury security as of that day. CMT rates are read from fixed points on the curve, so they may not match the yield of any single bond trading in the market.⁷

Both indices move with inflation expectations, Federal Reserve policy, and overall economic conditions. Tracking the one your loan uses gives you advance warning of where the next adjustment is headed.

What You’ll Pay Before and After the Fixed Period

The introductory rate is the number you’ll see advertised, but it’s not the number the lender uses to decide whether to approve you. The qualifying rate depends on how long the fixed period lasts:

Fixed period of three years or less: qualified at the maximum rate that could apply during the first five years.
Fixed period of five years: qualified at the greater of the maximum rate possible in the first five years (your note rate plus the first adjustment cap) or the fully indexed rate.
Fixed period longer than five years: qualified at least at the note rate, though higher-priced mortgage loans must use the greater of the note rate or the fully indexed rate.⁸

This is where the cap structure directly affects your purchasing power. A 5/1 ARM with a 2% initial cap and a 5.5% note rate means the lender qualifies you at 7.5%, not 5.5%. If the payment at 7.5% is more than you can carry, the attractive introductory rate won’t save the application.

Interest-Only ARMs

Some ARMs include an interest-only period of roughly three to ten years, during which your monthly payment covers interest only and none of the principal. Your loan balance doesn’t shrink at all during that window. When the interest-only phase ends, the payment jumps because you now owe both principal and interest, spread over fewer remaining years than the original loan term.⁹ That increase can be steep even if interest rates haven’t moved. Interest-only ARMs are not eligible as qualified mortgages under federal lending standards, so they receive less regulatory safe-harbor protection and lenders must more carefully document your ability to handle the eventual higher payment.¹⁰

Protections Built Into Every ARM

The Truth in Lending Act and its implementing regulation, Regulation Z, set the baseline consumer protections for ARMs.¹¹ A few of them are worth knowing by name before you shop.

Before you pay any non-refundable fee, or when you receive an application for a variable-rate loan secured by your home, the lender must give you a program disclosure identifying the index, the margin, how adjustments are calculated, and any caps on rate or payment changes.¹ Along with it comes the Consumer Handbook on Adjustable-Rate Mortgages (CHARM booklet), a CFPB publication explaining ARM risks in plain English. If you applied by phone or through a broker, the lender has three business days to get these materials to you.¹²

Federal rules also prohibit prepayment penalties on any loan whose annual percentage rate can increase after closing. Since every ARM’s rate can increase by definition, no ARM can carry a prepayment penalty.¹³ That protection matters because one of the smartest ARM strategies is refinancing into a fixed-rate loan before the first adjustment, and a penalty would undermine the entire reason for picking an ARM in the first place.

Negative amortization, where your payment doesn’t cover all the interest owed and your loan balance grows, cannot appear in a qualified mortgage at all.¹⁰ For the handful of non-qualified loans that still allow it, federal law requires the lender to warn you in writing before closing that your balance can grow and your equity will shrink.¹⁴

Getting Out: Refinance, Conversion, and Rescission

Because ARMs carry no prepayment penalty, you can refinance into a fixed-rate loan at any point without being charged for the exit itself. The friction is the cost and qualification process of the new loan, not the old one.

Some ARM contracts also include a conversion clause that lets you switch to a fixed rate without a full refinance. For a loan to be eligible for delivery to Fannie Mae as a converted ARM, it must be at least 12 months old, payments must be current, and the loan-to-value ratio at conversion cannot exceed the maximum limits for fixed-rate mortgages. The lender executes a loan modification agreement rather than originating a new mortgage.¹⁵ Not every ARM offers this, and conversion typically comes with a fee. If your loan documents don’t include a convertibility provision, your path to a fixed rate is a standard refinance.

If you’re refinancing an existing mortgage into an ARM rather than buying a home, federal law gives you three business days to cancel the deal after signing. The clock starts once you’ve signed the promissory note, received the Truth in Lending disclosure, and received two copies of a notice explaining your right to rescind. Saturdays count as business days for this purpose; Sundays and federal holidays don’t.¹⁶ This right does not apply to a purchase mortgage; once you sign closing documents on a home purchase, the transaction is final. If the lender failed to provide the required disclosures or rescission notice on a refinance, the cancellation window can extend up to three years from closing.

1
eCFR. 12 CFR 1026.19 – Certain Mortgage and Variable-Rate Transactions
2
Consumer Financial Protection Bureau. Consumer Handbook on Adjustable-Rate Mortgages
3
U.S. Department of Housing and Urban Development. FHA Adjustable Rate Mortgage
4
Consumer Financial Protection Bureau. What Are Rate Caps With an Adjustable-Rate Mortgage (ARM), and How Do They Work
5
Federal Reserve Bank of New York. Secured Overnight Financing Rate Data
6
Federal Register. Adjustable Rate Mortgages: Transitioning From LIBOR to Alternate Indices
7
U.S. Department of the Treasury. Interest Rates Frequently Asked Questions
8
Fannie Mae. Qualifying Payment Requirements
9
Office of the Comptroller of the Currency. Interest-Only Mortgage Payments and Payment-Option ARMs
10
GovInfo. 15 USC 1639c – Minimum Standards for Residential Mortgage Loans
11
National Credit Union Administration. Truth in Lending Act (Regulation Z)
12
GovInfo. Notice of Availability of Revised Consumer Information Publication
13
eCFR. 12 CFR 1026.43 – Minimum Standards for Transactions Secured by a Dwelling
14
Office of the Law Revision Counsel. 15 USC 1639c – Minimum Standards for Residential Mortgage Loans
15
Fannie Mae. Convertible ARMs
16
Consumer Financial Protection Bureau. How Long Do I Have to Rescind? When Does the Right of Rescission Start