Short answer
A fixed interest rate stays the same; a variable interest rate can change. In mortgages, the CFPB calls the latter an adjustable rate, and most adjustable-rate mortgages (ARMs) start with a fixed period. The signed note is the place to look for language governing rate changes—not just the rate currently being charged. 1 2
On this page
At a glance
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| Question | Fixed interest rate | Variable interest rate |
|---|---|---|
| Can the mortgage rate change? | No | Yes |
| Can it start out fixed? | It remains fixed | Most ARMs have an initial fixed period |
| Principal and interest payment? | Stays the same | Can change as the rate changes |
| Where to check the structure? | Loan disclosures and signed note | Loan disclosures and signed note |
What each thing is
These labels describe the interest-rate structure, not the loan’s repayment term or program category. The CFPB treats those as separate mortgage characteristics. A fixed-rate mortgage keeps its rate unchanged; an ARM allows later adjustments, usually after an introductory period. Here, “variable” is the comparison label for the adjustable mortgage structure. 1
Key differences
The central difference is future rate behavior. After an ARM’s initial fixed period, its rate can rise or fall regularly with market changes, and its principal and interest payment can change. A fixed-rate mortgage lacks that rate-change exposure. Some ARMs have caps, so adjustable does not necessarily mean unlimited adjustment. 1
How to tell them apart
Use the loan documents rather than a run of identical payments. The signed Promissory Note may identify an adjustable rate and point to the section controlling changes. The CFPB’s August 2024 guidance directs mortgage applicants after October 3, 2015, to page 1 of the Closing Disclosure; earlier applications and specified exceptions use Truth in Lending disclosures. 2
Where they overlap
Both structures can produce a period of unchanged interest charges. Most ARMs begin with a rate that does not change during the introductory period, just as a fixed-rate mortgage’s rate remains unchanged. That temporary similarity does not make their long-term structures identical: the ARM has a later adjustment period. 1
Edge cases
A fixed-rate mortgage’s total monthly payment can change even though its rate and principal and interest payment stay the same. Property taxes, homeowners insurance, or mortgage insurance can account for the difference. A payment increase alone therefore cannot establish that the interest rate is variable. 1
Why the distinction exists
The distinction separates rate stability from exposure to later market changes. It helps explain why today’s rate does not tell the whole story of an ARM. To understand when changes are permitted, examine the rate-change section identified in the signed note; the supplied excerpts do not spell out its detailed adjustment formula. 1 2
Common misconceptions
“Variable” does not mean the rate must change immediately, and “fixed” does not mean the total payment can never change. Nor does “adjustable” mean the rate only moves upward: the CFPB describes both increases and decreases. Keep the rate, principal and interest payment, and total payment distinct. 1
Examples
Two hypothetical mortgage cases illustrate the boundary. First, a borrower’s fixed-rate payment rises because homeowners insurance increases: the rate remains fixed. Second, a borrower’s ARM rate stays unchanged throughout its introductory period: the loan remains adjustable because it allows later changes. Neither the latest payment nor the current rate alone settles the classification. 1