Variable mortgage rates, more formally called adjustable-rate mortgages, are based on two numbers added together: a published benchmark index that moves with financial markets, and a fixed margin the lender sets when you close. The index changes over time in response to Federal Reserve policy and broader economic conditions, so your interest rate, and your monthly payment, can rise or fall at the adjustment dates written into your loan.
Index Plus Margin Equals Your Rate
Every adjustable-rate mortgage uses the same basic formula. The index is a published interest rate that fluctuates with general market conditions. The margin is a fixed number of percentage points the lender adds on top. Your lender locks in the margin at closing and it never changes for the life of the loan. When the index rises, your rate rises by the same amount; when the index falls, your rate follows.1Consumer Financial Protection Bureau. For an Adjustable-Rate Mortgage (ARM), What Are the Index and Margin, and How Do They Work?
The sum is called the fully indexed rate. If the current index sits at 4.0% and your margin is 2.75%, your fully indexed rate is 6.75%. The margin reflects your credit risk and the lender’s profit target, so borrowers with stronger credit profiles can often negotiate a lower one. Because that number stays with you for the entire term, comparing margins across lenders is one of the most useful ways to reduce the long-term cost of the loan. The index, by contrast, is not something the lender controls; it is set by the market.
The Benchmarks Lenders Actually Use
Secured Overnight Financing Rate
SOFR is the dominant benchmark for adjustable-rate mortgages originated today. It measures the cost of borrowing cash overnight using U.S. Treasury securities as collateral, and the Federal Reserve Bank of New York publishes it daily.2Federal Reserve Bank of New York. Secured Overnight Financing Rate Because SOFR is calculated from actual overnight lending transactions rather than estimates or surveys, regulators consider it a more reliable and transparent measure of borrowing costs than the benchmark it replaced.
SOFR formally replaced the London Interbank Offered Rate (LIBOR) for newly originated adjustable-rate mortgages. HUD finalized that transition in a March 2023 rule, and existing LIBOR-indexed mortgages were required to switch to a spread-adjusted SOFR replacement by their next rate adjustment date after June 30, 2023.3Federal Register. Adjustable Rate Mortgages: Transitioning From LIBOR to Alternate Indices If you took out an ARM years ago that once referenced LIBOR, your loan documents should now identify the SOFR-based replacement index your servicer uses.
U.S. Prime Rate
The Prime Rate is the baseline rate commercial banks extend to their most creditworthy business clients. It typically runs about three percentage points above the federal funds rate and tends to move in step with Fed policy changes. Prime shows up more often in home equity lines of credit than in closed-end mortgages, but some ARMs still reference it.
Why the Index Moves
The Federal Open Market Committee sets a target range for the federal funds rate, which as of early 2026 has an upper limit of 3.75%.4Federal Reserve Bank of St. Louis. Federal Funds Target Range – Upper Limit That target influences what banks charge each other for overnight lending, and the effect ripples into consumer lending. When the Fed raises its target, Prime almost always moves by the same amount within days. SOFR tracks closely too, because it measures the secured overnight lending market that Fed liquidity operations affect directly.
In practical terms, a quarter-point Fed hike translates into roughly a quarter-point increase in most ARM indices. You will not see it in your payment right away, though. The change only hits when your loan reaches its next scheduled reset. A Fed move today does nothing to a 5/1 ARM still in its fixed period; the first adjustment after that period ends is where the accumulated index changes finally show up.
The Fed itself is watching economic data. The Consumer Price Index, published monthly by the Bureau of Labor Statistics, measures the average change over time in prices paid by consumers for a basket of goods and services.5U.S. Bureau of Labor Statistics. Consumer Price Index Persistent inflation pushes the Fed toward higher rates to cool spending; inflation falling back toward the 2% target makes rate cuts more likely. Employment reports and GDP growth feed in as well, as does the 10-year Treasury yield, which reflects what bond investors expect from inflation and growth over the coming decade. No single indicator dictates where your rate will go, but together they describe the environment driving your next adjustment.
Fixed Periods and the First Adjustment
Most adjustable-rate mortgages sold today are hybrids that start with a fixed-rate period before switching to periodic adjustments. The two numbers in the product name describe that structure: a 5/1 ARM holds the rate steady for five years and then adjusts once a year, a 7/1 ARM is fixed for seven, and a 10/1 ARM for ten. FHA-insured ARMs are available with initial fixed periods of 1, 3, 5, 7, or 10 years.6U.S. Department of Housing and Urban Development. FHA Adjustable Rate Mortgage
During the fixed period, many lenders offer a discounted “teaser” rate that sits below the fully indexed rate. It attracts borrowers and can produce real savings if you sell or refinance before the fixed window closes. Once the initial period ends, the rate resets to the current index plus your margin, subject to any caps in the agreement.1Consumer Financial Protection Bureau. For an Adjustable-Rate Mortgage (ARM), What Are the Index and Margin, and How Do They Work? That first reset is where borrowers most often experience payment shock, especially when the teaser was well below the fully indexed level.
Rate Caps Limit How Far Your Rate Can Travel
The index and margin determine what your rate would be in raw form, but caps determine what it can actually become. Most ARMs include three:7Consumer Financial Protection Bureau. What Are Rate Caps With an Adjustable-Rate Mortgage (ARM), and How Do They Work?
- An initial adjustment cap, which limits how much the rate can change the first time it adjusts after the fixed period expires. Two or five percentage points is common.
- A subsequent adjustment cap, which limits the change at each reset after the first, usually one or two percentage points.
- A lifetime cap, which limits the total increase or decrease over the whole loan term. Five percentage points is typical.
FHA cap structures are set by product. A 5-year FHA ARM may allow up to one point annually with a five-point lifetime cap, or up to two points annually with a six-point lifetime cap. Seven-year and 10-year FHA ARMs are limited to two-point annual increases and six points over the life of the loan.6U.S. Department of Housing and Urban Development. FHA Adjustable Rate Mortgage Multiplying your caps against your starting rate gives you the worst-case payment before you sign.
A Boundary Worth Knowing: Payment Caps and Negative Amortization
Some ARMs include payment caps in addition to rate caps. A payment cap limits how much your monthly payment can rise at each adjustment. That sounds protective, but if the capped payment falls short of the interest actually accruing, the unpaid interest gets added to your loan balance and you end up owing more than you borrowed.8Consumer Financial Protection Bureau. Consumer Handbook on Adjustable Rate Mortgages This is called negative amortization. Payment-option ARMs, which let borrowers choose between full payments and reduced minimums, are the products most prone to it. If a loan offer mentions payment caps or minimum payment options, ask the lender to walk you through the scenario where the balance grows instead of shrinking, because a rate cap and a payment cap are not the same protection.