The Two Phases of an ARM
Every adjustable-rate mortgage has two distinct phases. During the initial fixed period, the interest rate does not change at all — it behaves exactly like a fixed-rate loan. Common initial periods are three, five, seven, or ten years. A loan labeled a 7/6 ARM, for example, holds its starting rate for seven years, then recalculates every six months.
Once that initial window closes, the loan enters the adjustment phase. At each scheduled adjustment date, the lender recalculates your rate by combining two numbers: the current value of a published benchmark index and a fixed margin set when you closed the loan. That sum becomes your new interest rate for the next adjustment period.
For a broader look at how fixed and adjustable structures compare, see how fixed and adjustable rates differ.
2/2/5
Most common ARM cap structure in the U.S.
This cap structure — limiting the first adjustment to 2%, each subsequent adjustment to 2%, and the lifetime increase to 5% — is the standard configuration disclosed in most conforming ARM products.
60–120 days
Required advance notice before an ARM rate adjusts
Federal Regulation Z mandates lenders provide written notice of an upcoming rate adjustment within this window, including the new rate and payment amount.
Index, Margin, and How Your Rate Is Calculated
The index is a market-based interest rate your lender does not control. In the U.S., most ARMs originated after 2023 use SOFR. The index moves up and down based on broader credit market conditions. Because lenders cannot dictate it, the index is the variable element of your rate.
The margin is the lender's fixed markup, typically between 2% and 3%, added on top of the index. It stays constant for the life of the loan. If SOFR is at 4.5% and your margin is 2.5%, your fully indexed rate is 7%. That is the rate your lender uses — subject to caps — before calculating your new payment.
Understanding how benchmark rate moves ripple through variable-rate products is useful context here. Our explainer on how interest rate changes affect savings and debt covers that broader picture.
Ask for the Worst-Case Payment Scenario
Before signing an ARM, ask your lender to calculate the maximum possible payment assuming the rate hits the lifetime cap. This payment stress test helps you judge whether you could still afford the home if rates rise to the legal ceiling. Most lenders are required to disclose this figure, and it is also included in the ARM disclosure form provided at application.
Rate Caps: The Limits on How Far Your Rate Can Move
Rate caps are the most important consumer protection built into an ARM. Most ARM products carry a three-part cap structure, often written as three numbers separated by slashes — for example, 2/2/5.
- Initial cap (first number): The maximum the rate can rise at the very first adjustment. A 2% initial cap means if your start rate was 5%, it cannot exceed 7% at that first change, regardless of where the index sits.
- Periodic cap (second number): The maximum increase (or decrease) allowed at any single subsequent adjustment. A 2% periodic cap limits each annual change to no more than 2 percentage points.
- Lifetime cap (third number): The maximum total increase over the life of the loan from the initial rate. A 5% lifetime cap means a loan that started at 5% can never exceed 10%, no matter how high the index climbs.
These caps are disclosed in your Loan Estimate before closing and detailed in your loan's promissory Note. Reviewing both documents carefully is essential. For definitions of these and related terms, our mortgage terminology reference is a useful starting point.
When Adjustments Happen and What to Expect
Your loan documents specify exact adjustment dates — they are not at the lender's discretion. Federal rules require lenders to mail an adjustment notice between 60 and 210 days before the first adjustment, and between 60 and 120 days before each subsequent one. That notice must show the new interest rate, the resulting payment, and a plain explanation of how the rate was determined.
If you are weighing whether to lock a rate before closing on an ARM, how rate locks work explains the mechanics. And if you are still deciding between loan structures altogether, see comparing ARM and fixed-rate structures for a decision-oriented comparison.
This article is for general informational purposes only and does not constitute financial, mortgage, or legal advice. Mortgage products, rates, and regulations vary. Consult a licensed mortgage professional for guidance tailored to your specific situation.
