Adjustable-rate mortgage (ARM)

A mortgage whose interest rate is fixed for an initial period and then adjusts periodically based on a market index plus a margin.

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Primary references on this page:

CFPB: ARM index, margin, and fully indexed rate

CFPB: adjustable-rate mortgage caps

An adjustable-rate mortgage (ARM) starts with a fixed interest rate for an introductory period, then resets on a schedule for the rest of the loan. The name usually encodes the structure: a "5/6 ARM" is fixed for five years and then adjusts every six months. After the fixed period, the new rate is calculated from a published market index plus a fixed margin set in your loan documents, usually subject to caps that limit how much the rate can move at each adjustment and over the life of the loan.

The trade is straightforward: the introductory rate is often lower than a comparable fixed-rate loan, in exchange for uncertainty later. ARMs can make sense when you expect to sell or refinance before the first reset, and they can hurt when rates rise and you are still holding. For a rental, the question is whether the property's cash flow survives the worst-case rate your caps allow — not whether the teaser rate looks good today.

Reset mechanics come from the contract

An ARM commonly has an initial rate period followed by scheduled adjustments. At a reset, the referenced index plus the contractual margin produces the fully indexed rate, then the note’s initial, periodic, lifetime, and floor provisions constrain the result. Payment recalculation also depends on remaining balance, remaining term, and any payment-cap or negative-amortization provisions.

Suppose the initial note rate is 5.00%, the reset index is 4.25%, and the margin is 2.25%. The fully indexed rate is 6.50%. If the first-adjustment cap permits at most a 1.00 percentage-point increase from 5.00%, this reset would be 6.00%, not 6.50%. A different contract could produce a different result; the note, rider, and current notice control.

Illustrative rate-reset calculation
Step	Result	Control
Index + margin	4.25% + 2.25% = 6.50%	Fully indexed rate
Initial rate + hypothetical cap	5.00% + 1.00% = 6.00%	Maximum first reset in this example
Constrained reset rate	6.00%	Lower of fully indexed and capped rate

Stress the payment, coverage, and cash-flow consequences

Before comparing an ARM, record the first reset date, index, observation date, margin, adjustment frequency, each cap, lifetime ceiling and floor, payment cap, negative-amortization possibility, remaining amortization term, and balance expected at reset. Then recalculate debt service and test DSCR and cash flow at the contractual reset and ceiling cases.

An ARM’s initial rate does not predict later payments. Refinancing before reset is not assured, an index can change, and a payment cap may defer rather than eliminate interest. CFPB guidance confirms that index plus margin is subject to caps; the signed contract determines the actual mechanics.

Related tools & guides

Mortgage calculator

Editorial ownership

Written and maintained by the Aptoria editorial team

Editorial method reviewed July 28, 2026. Aptoria reviews scope, source fit, examples, limitations, links, and publication gates. This record does not claim attorney, CPA, lender, appraiser, or other independent professional sign-off.

Professional review is not claimed. Verify current law, tax treatment, loan terms, valuation inputs, and property-specific facts with the appropriate qualified professional before acting.

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Primary and authoritative sources

CFPB: ARM index, margin, and fully indexed rate ↗

CFPB: adjustable-rate mortgage caps ↗