SOFR Explained: How the Benchmark Rate Behind Trillions in Loans Actually Works

If you have an adjustable-rate mortgage, a business line of credit, or certain student loans originated in recent years, there is a good chance your interest rate is tied to a number called SOFR. For decades that role belonged to LIBOR, the London Interbank Offered Rate. LIBOR was phased out after regulators found that some banks had manipulated it, and SOFR has since become the dominant U.S. reference rate for loans and derivatives.

What SOFR Actually Measures

SOFR stands for the Secured Overnight Financing Rate. It reflects the cost of borrowing cash overnight using U.S. Treasury securities as collateral, a market known as repurchase agreements, or repo. Each business day, the Federal Reserve Bank of New York calculates SOFR using data from a large volume of actual overnight repo transactions, not estimates or surveys. The New York Fed publishes the rate on its website, along with historical data and methodology details.

Because SOFR is built from real transactions backed by Treasury collateral, it is considered a secured and nearly risk-free rate. That is a key structural difference from LIBOR, which was based on unsecured interbank lending and relied partly on self-reported estimates from a panel of banks rather than a deep pool of verifiable trades.

Why LIBOR Was Replaced

LIBOR underpinned hundreds of trillions of dollars in contracts worldwide, from mortgages to complex derivatives. In the early 2010s, investigations by regulators including the U.S. Commodity Futures Trading Commission and UK authorities revealed that traders at several banks had attempted to manipulate LIBOR submissions to benefit their trading positions. The scandal exposed a structural weakness: LIBOR depended on a shrinking number of actual unsecured interbank loans, making it vulnerable to manipulation and eventually unsustainable as a benchmark.

In response, regulators and central banks formed working groups to identify alternative reference rates. In the United States, the Alternative Reference Rates Committee, convened by the Federal Reserve, selected SOFR as its recommended replacement. U.S. dollar LIBOR panels ceased publication by mid-2023, with legacy contracts transitioning under rules set by the Adjustable Interest Rate (LIBOR) Act and related federal guidance.

The Different Flavors of SOFR

SOFR is not a single, static number used identically in every contract. Lenders and derivatives markets use several variations:

• Overnight SOFR: the raw daily published rate reflecting that day's overnight repo activity.
• Compounded SOFR in arrears: an average of daily SOFR rates compounded over a period, such as 30 or 90 days, calculated after that period ends. This is common in business loans and floating-rate notes.
• Term SOFR: a forward-looking rate for 1-, 3-, or 6-month periods, published by CME Group and endorsed by the Alternative Reference Rates Committee for certain cash products such as business loans, where knowing the rate in advance is operationally useful.
• SOFR averages: simple daily averages over 30, 90, or 180 days, published by the New York Fed for straightforward reference in consumer products.

The method a lender uses matters because it affects both the timing of rate resets and the final number a borrower pays. Compounded-in-arrears structures mean the exact payment amount for a period may not be known until near its end, which is a meaningful change in mechanics from the forward-looking LIBOR borrowers were used to.

Why SOFR-Based Rates Often Need an Adjustment

Because SOFR is a secured, risk-free rate, it generally runs lower than unsecured LIBOR did, since LIBOR embedded a credit risk premium reflecting the chance a bank might default. To bridge that gap for existing contracts transitioning away from LIBOR, regulators and market participants adopted fixed spread adjustments, published by organizations such as Bloomberg Index Services, so that legacy loans and bonds would not see an artificial windfall or shortfall purely from the benchmark switch. New contracts originated directly in SOFR typically build an appropriate margin into the loan's pricing from the start rather than relying on a separate transition adjustment.

What This Means for Borrowers and Investors

For everyday borrowers, the practical takeaway is to check loan or credit agreement disclosures for which SOFR variant applies, how often the rate resets, and whether a credit spread adjustment is included. For investors in floating-rate bonds, business development companies, or securitizations, understanding whether a security references term SOFR versus compounded SOFR in arrears can affect cash flow timing and valuation, particularly during periods of changing Federal Reserve policy rates, since SOFR tends to move closely with the federal funds rate even though it is a separate, market-determined benchmark.