The difference in a fixed vs adjustable rate mortgage is what the contract allows to change. In the Consumer Financial Protection Bureau's (CFPB) words, a fixed-rate mortgage has an interest rate that "is set when you take out the loan and it will not change during the term of the loan." An adjustable-rate mortgage (ARM) has a rate that "may go up or down." At each reset, the ARM's new rate is the index plus a fixed margin, held inside the caps written into the loan.
This guide walks through what actually happens on a reset date, step by step, using the terms the federal disclosure rules require lenders to print. It is general information about US mortgage contracts, not advice about any loan or about which type suits any household.
Fixed vs adjustable rate mortgage: the core difference
Both loans are usually repaid in monthly installments of principal and interest over a set term. The difference is the rate those installments are built on.
| Feature | Fixed-rate mortgage | Adjustable-rate mortgage (ARM) |
|---|---|---|
| Interest rate | Set at closing, unchanged for the term | Fixed for an initial period, then recalculated on a schedule |
| What sets a later rate | Nothing; there is no later rate | Index plus margin, limited by caps |
| Principal and interest payment | Same for the life of the loan | Can rise or fall at each adjustment |
| Federal reset notices | None needed for the rate | Required before payments change |
| Where the terms are shown | Loan Estimate and promissory note | Loan Estimate's adjustable interest rate table and the note |
The CFPB's page on the difference between the two (last reviewed January 14, 2025, read October 2, 2026) notes that many ARMs begin with a lower initial rate than a fixed loan, and that after the initial period the rate adjusts periodically, which often means a higher monthly payment. Whether that trade is worthwhile depends on facts about the borrower and the loan that no general article can weigh.
What "fixed" actually fixes
A fixed rate fixes the interest rate, and therefore the principal and interest portion of the payment. It does not fix the whole monthly bill.
Many mortgage payments also carry escrow for property taxes and homeowners insurance. When the county reassesses the home or the insurer changes the premium, the servicer recalculates the escrow portion, and the total payment moves even though the interest rate has not. The premium the servicer pays is the one printed on the policy, and our guide on how to read an insurance declarations page shows where to find it. When a fixed-rate payment changes, the escrow portion is the usual place to look first, because the rate itself cannot move.
The four numbers an ARM contract runs on
An ARM's reset is not a negotiation. It is arithmetic that the loan documents set out before closing, built from four inputs.
The index
The CFPB defines the index as "a benchmark interest rate that reflects general market conditions." The lender chooses which index the loan follows at application, and according to the CFPB's page on index and margin (last reviewed April 3, 2024, read October 2, 2026), that choice typically does not change after closing. The index is the only input that moves with the market.
The margin
The margin is "the number of percentage points added to the index by the mortgage lender to set your interest rate." It is fixed in the loan agreement and does not change after closing. The CFPB also notes that margins vary between lenders. Combined with different caps, that is why two ARMs with the same starting rate can behave very differently after the first reset.
The caps
Caps limit how far the rate can move. The CFPB's page What are rate caps with an adjustable-rate mortgage (ARM), and how do they work? (last reviewed January 14, 2025, read October 2, 2026) describes three:
- Initial adjustment cap: limits the first change. The CFPB says "it's common for this cap to be either two or five percent."
- Subsequent adjustment cap: limits each later change. "This cap is most commonly one or two percent."
- Lifetime adjustment cap: limits the total change from the initial rate. "This cap is most commonly five percent."
These are the CFPB's descriptions of common structures, not rules. The caps on a specific loan are whatever its documents say. The same page points out that "two different lenders may have the same initial interest rate but offer different rate caps."
The floor
Some ARMs also limit how low the rate can go. The CFPB notes that "some ARMs also limit how much your interest rate may decrease." The Loan Estimate shows this as the loan's minimum interest rate.
What happens on a reset date, step by step
When the initial fixed period ends, the contract runs the same sequence at the first reset and at every reset after it.
- Read the index. The note specifies which index value is used and as of what date. That value is taken as published.
- Add the margin. Index plus margin gives the rate the formula produces before any limits. The CFPB states this as "Index + Margin = Your Interest Rate," subject to caps.
- Apply the adjustment cap. At the first reset, the new rate cannot move further from the initial rate than the initial adjustment cap allows. At later resets, the subsequent cap limits the move from the previous rate.
- Apply the lifetime cap and the floor. The result cannot sit above the loan's maximum rate or below its minimum rate.
- Recalculate the payment. The new rate is applied to the remaining balance over the remaining term, which produces the new principal and interest payment.
The rate that survives all five steps is the rate the borrower pays until the next scheduled change.
A worked illustration
The figures below are invented round numbers chosen to show the mechanics. They are not typical rates, margins or caps, and they describe no real loan.
Suppose an ARM starts at 6.00% for its initial period, has a margin of 2.75 percentage points, an initial adjustment cap of 2, a subsequent cap of 1, and a lifetime cap of 5.
- First reset, index at 5.00%. Index plus margin is 7.75%. The initial cap allows up to 8.00%, so the new rate is 7.75%.
- Second reset, index at 6.50%. Index plus margin is 9.25%. The subsequent cap allows only one point above 7.75%, so the rate is held at 8.75%.
- Lifetime ceiling. No matter how high the index climbs, this loan's rate can never exceed 11.00%, which is the 6.00% start plus the 5-point lifetime cap.
The second reset shows something many comparisons leave out. When a cap holds the rate below the formula, the gap does not disappear. If the index stays high, the rate can keep stepping up at later resets until it reaches the formula rate or the lifetime maximum.
How the name of an ARM maps to the contract
ARMs are often advertised with two numbers. In the CFPB's glossary, a 5/1 ARM is one where "5" is "the number of years your initial interest rate will stay fixed." The second number generally describes how often the rate can change after that, although product naming is a marketing convention, not a disclosure.
The binding version sits on the Loan Estimate. Under 12 CFR 1026.37(j), an ARM's Loan Estimate must include an adjustable interest rate table showing:
- the index and the margin added to it,
- the initial interest rate,
- the minimum and maximum interest rates after any introductory period,
- when the rate may first change and how often it changes after that, and
- the limits on the first change and on later changes.
Comparing two ARMs line by line on that table shows far more than comparing their headline names or starting rates.
The notices that arrive before a reset
This is the part of the contract most borrowers meet for the first time when the letter arrives. Regulation Z, the federal rule that implements the Truth in Lending Act, requires advance warning before an ARM's payment changes. The CFPB publishes the text of 12 CFR 1026.20 (read October 2, 2026).
The first-reset notice. Section 1026.20(d) requires the initial rate adjustment notice "at least 210, but no more than 240, days before the first payment at the adjusted level is due." It must show the current and new rates (the new one may be an estimate), the current and new payments, how the rate is determined, the caps, and alternatives such as refinancing, sale, modification and forbearance. It must also point to housing counselor resources and give a phone number for the creditor or servicer.
Every later payment change. Section 1026.20(c) generally requires a notice "at least 60, but no more than 120, days before the first payment at the adjusted level is due." Shorter windows apply in specific cases, such as ARMs that adjust every 60 days or more often.
So a borrower with an ARM normally gets roughly seven to eight months' warning before the first new payment, and about two to four months before each later one. A fixed-rate borrower gets no such notice, because there is no rate change to announce.
Terms that change a payment beyond the rate
The rate is not the only term worth reading on either type of loan. The CFPB's glossary describes a few features that change how a payment behaves:
- Interest-only periods: "scheduled payments that require you to pay only the interest for a specified amount of time." The balance does not fall during that time.
- Negative amortization: "If payments are less than the amount of interest due each month, the mortgage balance will grow rather than decrease."
- Prepayment penalty: "a fee that some lenders charge if you pay off all or part of your mortgage early." On an ARM, this term matters if the loan might be refinanced or the home sold before a reset.
Whether any of these appear is stated in the loan documents and on the Loan Estimate.
Who regulates this, and who can help
Mortgages are regulated products. The CFPB writes and enforces the federal disclosure rules described above, and mortgage lenders and servicers are also licensed and supervised at the state level. The state agency that oversees mortgage lending can confirm whether a particular lender or servicer is licensed there.
For questions about a specific loan, the servicer is the first contact, and the first-reset notice must include its phone number. A HUD-approved housing counselor can walk through the notice and the alternatives it lists. A real estate attorney can explain the promissory note itself. Each will want the note, the Loan Estimate or Closing Disclosure, and the most recent rate adjustment notice. For a sense of what the insurance part of the escrow payment is buying, our explainer on homeowners insurance Coverage A through F breaks down each part of the policy.
Frequently asked questions
What is the main difference between a fixed and an adjustable rate mortgage?
A fixed-rate mortgage keeps the same interest rate for the whole term. An adjustable-rate mortgage keeps its initial rate for a set period, then recalculates the rate on a schedule using an index plus a margin, limited by caps.
How is an ARM's new rate calculated at reset?
The lender takes the current value of the loan's index, adds the margin fixed in the loan agreement, then limits the result with the adjustment cap, the lifetime cap and any floor. The CFPB summarizes this as index plus margin, subject to caps.
Can a fixed-rate mortgage payment go up?
The principal and interest portion cannot change. The total payment can, because property taxes and homeowners insurance paid through escrow are recalculated as those bills change.
How much notice is given before an ARM resets?
Under 12 CFR 1026.20(d), the first-reset notice is generally due 210 to 240 days before the first payment at the new rate. Later payment changes generally require a notice 60 to 120 days ahead, under section 1026.20(c).
What do the numbers in a 5/1 ARM mean?
The CFPB's glossary says the "5" is the number of years the initial rate stays fixed. The second number generally describes how often the rate can change after that. The loan's adjustable interest rate table shows the actual schedule.
Federal mortgage rules and CFPB guidance can change. The details in this article were checked against CFPB sources on October 2, 2026.
