Glossary term
Index (ARM)
An ARM index is the benchmark rate used with a margin to set an adjustable-rate mortgage's interest rate after the initial period.
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What Is an ARM Index?
An ARM index is the benchmark rate used to help set the interest rate on an adjustable-rate mortgage after the initial fixed-rate period ends. The lender adds the loan's margin to the index, then applies any caps or other contract limits.
The index matters because it moves with market conditions. If the index rises, the borrower's adjusted rate may rise. If the index falls, the rate may fall, depending on the loan terms, caps, floors, and timing.
Key Takeaways
- An ARM index is a benchmark used to calculate adjusted mortgage rates.
- The adjusted rate is generally based on the index plus the lender's margin.
- The borrower does not control the index selected in the loan documents.
- Caps and floors can limit how much the rate changes even when the index moves.
How an ARM Index Works
The ARM note identifies the index used for future adjustments. At each adjustment date, the lender looks at the current value of that index and adds the fixed margin. The result is often called the fully indexed rate, before any caps or rounding rules are applied.
Common ARM indexes have changed over time. Some older loans used indexes that are no longer common or have been discontinued. Borrowers should read the loan documents to know the exact index and replacement language.
Index, Margin, and Caps
Term | Role in the ARM rate |
|---|---|
Index | Market benchmark that can change over time |
Margin | Fixed percentage added to the index |
Fully indexed rate | Index plus margin before caps or rounding |
Periodic cap | Limits the change at a single adjustment |
Lifetime cap | Limits the maximum rate over the loan's life |
Borrower Review Points
Borrowers should identify the index, how often it is measured, how the rate is rounded, what replacement index applies if the original index is unavailable, and how caps interact with index movements.
The index is not the same as the borrower's rate. A low index can still produce a higher mortgage rate once the margin is added. A high index may not immediately pass through fully if caps limit the change.
It is also worth asking how transparent the index is. A borrower should be able to find the index value independently rather than relying only on the servicer's adjustment notice.
The Bottom Line
An ARM index is the benchmark used to reset an adjustable-rate mortgage after the initial period. It is only one part of the rate formula, so borrowers should evaluate it with the margin, caps, floors, and adjustment schedule.