Fixed vs Variable Rates on a Loan, Explained

A fixed rate is set at closing and stays the same for the life of the loan, so every scheduled payment of principal and interest is known in advance. A variable (adjustable) rate is recalculated on dates written into the contract as a published index plus a fixed margin, within caps that limit how far it can move. The real question is whether a variable loan's usually lower starting rate is worth payments that can change.

Key takeaways

Fannie Mae and Freddie Mac SOFR ARMs use the 30-day average SOFR and adjust every six months after a 3-, 5-, 7- or 10-year fixed period. Freddie Mac's caps are 2/1/5 on 3/6 and 5/6 ARMs and 5/1/5 on 7/6 and 10/6 ARMs.
On a 7/6 or 10/6 ARM the first-change cap equals the lifetime cap, so the rate can reach its ceiling at the first reset. In the illustrative $400,000 example below, that raises the payment from $2,594 to $3,815 (+47%).
Under the general qualified-mortgage rule, the lender underwrites at the highest rate possible in the first five years. On a 7/6 or 10/6 ARM that is the start rate, so the rule does not require a test of the post-reset payment.

What stays fixed and what floats

On a fixed-rate amortizing loan, the rate and the monthly principal-and-interest payment are set once. The payment stays level, so the share going to principal grows over time. If market rates fall, capturing them takes a refinance, which is never guaranteed.

On a variable-rate loan, the rate is recalculated on scheduled dates. The CFPB's Consumer Handbook on Adjustable-Rate Mortgages (the "CHARM booklet," January 2014 edition) describes the two parts: the index, a measure of interest rates generally, and the margin, the extra amount the lender adds. On a fully amortizing ARM, each change recalculates the payment to repay the remaining balance over the remaining term.

Where each structure shows up

Mortgages: 30- and 15-year fixed loans, and hybrid ARMs that hold a start rate for a set period before adjusting. Freddie Mac's Primary Mortgage Market Survey put the average 30-year fixed rate at 7.40% and the 15-year at 6.73% in the week of October 8, 2026.
Home equity lines of credit: the CFPB notes that HELOCs usually have a variable rate. A draw period (10 years, for example) is followed by repayment, when monthly payments are often significantly higher.
Credit cards: variable APRs are commonly tied to a published index, and prime is a frequent reference rate for card loans.
Federal student loans: Direct Loans first disbursed from July 1, 2026 to June 30, 2027 carry 6.52% (undergraduate), 8.07% (graduate unsubsidized) or 9.07% (PLUS). Federal Student Aid states that each loan's rate is fixed for its life.
Private student loans: either type. Regulation Z section 1026.47 requires the application disclosure to say which, and any limits on adjustments or that there are none.

The index-plus-margin formula

Every variable rate is built the same way. Only the index and the rules around it differ:

new rate = index value on the lookback date + margin
           then limited by the period cap, lifetime cap and floor

index    = a published market rate the lender does not control
margin   = percentage points fixed in the contract
lookback = how far before the change date the index is read
           (45 days on GSE SOFR ARMs)

The CHARM booklet's own example uses a 4% index plus a 3% margin, giving a 7% "fully indexed rate." If the index rises to 5%, the next rate is 8%. A start rate below the fully indexed rate is a "teaser," and the next reset can raise the payment even if the index never moves.

SOFR, the mortgage index

The Secured Overnight Financing Rate measures overnight borrowing costs backed by Treasury securities. The New York Fed publishes 30-, 90- and 180-day SOFR averages, compounded over rolling calendar periods, each business day. Freddie Mac's SOFR ARM fact sheet says both Freddie Mac and Fannie Mae require the 30-day average for newly originated single-family SOFR ARMs, with margins of 100 to 300 basis points (1 to 3 percentage points). The 30-day average SOFR stood at 3.86% on October 14, 2026. Older loans may use other indexes; the one named in the note governs.

Prime, the HELOC and card index

The prime rate is set by individual banks, not by the Fed. The Fed's FAQ says many banks base it partly on the federal funds target, and its H.15 release reports the rate posted by a majority of the 25 largest banks. That rate was 7.00% on October 2, 2026, the latest daily reading, against a federal funds target ceiling of 4.00% since September 17, 2026. The 3-point gap is a convention, not a rule, but across 4,497 days with both series published (December 16, 2008 to October 2, 2026) it failed on only three, each a one-day lag around a Fed decision.

Caps, floors and conversion terms

Caps are usually written as three numbers: first-change cap / later per-change cap / lifetime cap above the start rate. By law, the CHARM booklet notes, virtually all ARMs must have a lifetime cap. Freddie Mac's SOFR ARM product announcement (for loans settling from December 1, 2020) sets these terms:

Product	Initial fixed period	Adjusts every	Caps (first / later / lifetime)	Highest rate if the start rate is 6.75%
3/6 ARM	36 months	6 months	2 / 1 / 5	8.75% at first change, 11.75% lifetime
5/6 ARM	60 months	6 months	2 / 1 / 5	8.75% at first change, 11.75% lifetime
7/6 ARM	84 months	6 months	5 / 1 / 5	11.75% at first change, 11.75% lifetime
10/6 ARM	120 months	6 months	5 / 1 / 5	11.75% at first change, 11.75% lifetime

The last column follows mechanically from the caps; it is not a forecast. A 7/6 borrower trades a longer fixed period for less protection at the first reset.

Floors: how low the rate can go

Caps cut both ways. Fannie Mae's Selling Guide section B2-1.4-02 (December 10, 2025) says per-adjustment caps apply to decreases as well as increases, lifetime caps apply only to increases, and the rate may never fall below the margin. On a loan with a 2.75% margin, the floor is 2.75% however far the index drops. Other loans set their floor in the note, and some ARMs never adjust downward.

Conversion options

Some ARMs let the borrower switch to a fixed rate at set times, using a formula in the loan documents; the CHARM booklet notes these may carry a somewhat higher rate or up-front fees, plus a fee at conversion. Freddie Mac's SOFR ARM product page lists its 3/6, 5/6, 7/6 and 10/6 SOFR ARMs as nonconvertible, so leaving one means refinancing, which depends on income, home value and rates at that time.

Stress-testing a variable payment

A stress test replays the loan under the contract's own limits instead of guessing where rates go. Each reset payment comes from the standard amortization formula:

M = B × r / (1 − (1 + r)^(−n))

M = new monthly principal-and-interest payment
B = loan balance on the reset date
r = new annual rate ÷ 12
n = months left in the term

payment shock (%) = (new payment − old payment) ÷ old payment × 100

How much cheaper the start rate has been

Across the 932 weeks with both 30-year fixed and 5/1 ARM averages (January 6, 2005 to November 10, 2022), the ARM's initial rate averaged 0.66 points below the fixed rate (median 0.68). It peaked at 1.51 points (December 19, 2013), averaged 0.04 in 2020, and in 48 weeks the ARM was dearer. Freddie Mac stopped the ARM survey in November 2022, so these figures describe older 5/1 ARMs, not today's SOFR products.

Illustrative example: $400,000, 7/6 ARM vs 30-year fixed

Assumptions, not offers: a 30-year fixed loan at 7.40% (the October 8, 2026 survey average) and a 7/6 ARM starting at 6.75%, 0.65 points lower and close to the historical average gap. The ARM has a 2.75% margin and 5/1/5 caps; with 30-day average SOFR at 3.86%, the fully indexed rate is 6.61%. Two paths after month 84: the index stays at 3.86%, so the rate resets down to 6.61%; or the index reaches 9.00% or more, so the rate hits the 11.75% cap at the first change and stays there.

Measure	Fixed 7.40%	7/6 ARM, index unchanged	7/6 ARM, worst case
Monthly payment, months 1–84	$2,769.52	$2,594.39	$2,594.39
Monthly payment from month 85	$2,769.52	$2,563.11 (6.61%)	$3,814.97 (11.75%)
Interest paid, first 7 years	$199,440	$181,079	$181,079
Balance after 7 years	$366,801	$363,150	$363,150
Interest paid, 10 years	$278,753	$251,011	$307,298
Balance after 10 years	$346,411	$340,810	$352,030

If the home is sold at the end of year 7, the ARM comes out $18,362 ahead under any rate path, before any difference in closing costs: $14,711 in lower payments ($175.13 a month for 84 months) plus a balance $3,651 smaller. Held to year 10, the unchanged path saves $27,742 in interest, while the worst case costs $28,545 more, with cumulative interest overtaking the fixed loan's in month 99. The break-even rate from month 85 is about 9.15%, which needs a 30-day SOFR of about 6.40% with this margin. That is not a forecast, only a level the caps allow.

Illustrative example: payment shock at the first change

Same $400,000 loan and 6.75% start rate. Under the 7/6's 5-point first cap, the worst-case payment rises from $2,594.39 to $3,814.97, an increase of $1,220.58 or 47.0%. A 5/6 ARM with 2/1/5 caps limits the first step to 8.75%: the payment moves to $3,087.17 in month 61, a 19.0% shock, then can climb to $3,343.33, $3,604.03 and finally $3,868.59 at 11.75% from month 79. The smaller first cap spreads the increase out but reaches the same rate ceiling.

Illustrative example: a $50,000 HELOC at prime + 1

Assume a $50,000 balance with interest-only payments during the draw period and interest worked out as rate ÷ 12. Real bills can differ slightly, for example where interest is charged daily.

Prime rate	HELOC rate (prime + 1)	Monthly interest-only payment	Change vs today
5.00%	6.00%	$250.00	−$83.33
7.00% (October 2026)	8.00%	$333.33	—
9.00%	10.00%	$416.67	+$83.33

Each 1-point move in prime changes this payment by $41.67 a month, or 12.5% of today's payment, from the plan's next rate-change date. The bigger jump usually comes when the draw period ends and principal repayment begins.

What the lender must disclose

Regulation Z (12 CFR part 1026) sets what a borrower sees, and when, on home-secured variable-rate credit:

At application (closed-end ARMs): section 1026.19(b) covers variable-rate loans with terms over one year secured by the borrower's principal dwelling. When an application form is provided or before a nonrefundable fee is paid, whichever comes first, the lender must give the CHARM booklet and a program disclosure. It names the index and its source, how often and how far the rate can change, and shows a 15-year historical example or the maximum rate and payment on a $10,000 loan.
Before the first change: for closed-end ARMs secured by the principal dwelling, section 1026.20(d) requires a notice 210 to 240 days before the first payment at the adjusted rate is due, or at closing if that payment falls within 210 days. ARMs with terms of one year or less are exempt.
Before each payment change: section 1026.20(c) requires a notice 60 to 120 days before the first payment at a new level is due, or 25 to 120 days for ARMs that adjust every 60 days or more often.
Underwriting: under section 1026.43(e)(2)(iv), a general qualified mortgage is underwritten with the maximum rate that can apply during the first five years after the first payment is due. No change can occur in that window on a 7/6 or 10/6 ARM, so the test uses the start rate.
HELOCs: section 1026.40 requires, at application, the index and its source, any limits on rate changes, the maximum APR, the minimum payment at that rate on a $10,000 balance and a 15-year history of index values. The 1026.20 notices do not apply, because a HELOC is open-end credit.

Frequently asked questions

What happened to loans that were tied to LIBOR?

US dollar LIBOR panels ended after June 30, 2023. Contracts that named a replacement in their fallback language switched to it. For contracts without a workable fallback, the Federal Reserve Board's rule under the LIBOR Act named SOFR-based replacements for overnight, one-, three-, six- and 12-month LIBOR.

Are any federal student loans variable-rate?

Older ones can be. The Edfinancial servicer page on studentaid.gov states that federal student loans first disbursed from July 1, 1998 through June 30, 2006 have variable rates adjusted each July 1, while loans first disbursed on or after July 1, 2006 have a fixed rate for the life of the loan.

Can part of a HELOC balance be moved to a fixed rate?

Sometimes. The CFPB notes that some HELOCs let the borrower convert some or all of the balance to a fixed rate, which is usually higher than the variable rate but more predictable.

Must a card issuer warn before a variable APR rises?

Not when the rise simply follows the index. Under Regulation Z section 1026.9(c)(2), most significant changes to credit card terms need 45 days' written notice, but the rule exempts a variable-APR increase that follows a publicly available index outside the issuer's control, as the account agreement provides.

Can a HELOC lender change the index or margin later?

Generally not. Section 1026.40(f) allows rate changes only through an index the creditor does not control and the public can see. After the plan is opened, the creditor generally may not change its terms; one exception allows replacing an index that is no longer available with a comparable one.

Weighing the two before signing

Neither structure is better in general. The variable side is a bet on the holding period and on the index, and its worst case is written in the contract:

Consideration	Leans toward fixed	Leans toward variable
Expected time with the loan	Likely to keep it past the initial period	Firm plan to sell or repay before the first change
Payment tolerance	Budget cannot absorb the capped worst-case payment	Worst-case payment is affordable today
Start-rate discount	Small gap between ARM and fixed quotes	Gap large enough to matter over the fixed period
Exit plan	Relies on refinancing later, which needs income, equity and credit at that time	Exit does not depend on refinancing

Before signing a variable-rate loan, pull these items from the program disclosure and the note:

The index, its source, current value and lookback period.
The margin, today's fully indexed rate, and whether the start rate is below it.
The first, later and lifetime caps, and the floor.
The payment at the lifetime cap on the balance expected at the first reset, and its shock percentage against monthly income after other fixed costs.
Any conversion option and its cost; for a HELOC, when the draw period ends and how repayment is scheduled.