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SOFR Overnight Rate: Calculation, Compounding, and Variants
SOFR is the Secured Overnight Financing Rate, a volume-weighted median of overnight Treasury repo transactions published each US business day. It is the primary reference rate for US dollar derivatives and floating-rate debt, with daily underlying volume of roughly $2 trillion.
Key Takeaways
- SOFR is derived from a volume-weighted median of roughly one million individual overnight Treasury repo trades, making it nearly impossible to manipulate compared to LIBOR's panel submissions.
- September 2019 showed SOFR is not perfectly stable: it printed 5.25% intraday against an IORB of 2.2% when $100 billion drained from reserves, leading directly to the Standing Repo Facility.
- Investors using simple averages instead of daily compounding will systematically misprice floating-rate payments, especially in volatile rate environments, the compounding formula is contractually specified.
- Term SOFR (forward-looking, set by CME) and compounded SOFR in arrears (backward-looking, known only at period end) are not interchangeable, mixing them creates cash-flow timing mismatches in hedged portfolios.
Key Takeaways
- SOFR is derived from a volume-weighted median of roughly one million individual overnight Treasury repo trades, making it nearly impossible to manipulate compared to LIBOR's panel submissions.
- September 2019 showed SOFR is not perfectly stable: it printed 5.25% intraday against an IORB of 2.2% when $100 billion drained from reserves, leading directly to the Standing Repo Facility.
- Investors using simple averages instead of daily compounding will systematically misprice floating-rate payments, especially in volatile rate environments, the compounding formula is contractually specified.
- Term SOFR (forward-looking, set by CME) and compounded SOFR in arrears (backward-looking, known only at period end) are not interchangeable, mixing them creates cash-flow timing mismatches in hedged portfolios.
What It Is
SOFR is a broad measure of the cost of borrowing cash overnight collateralized by US Treasury securities. The Federal Reserve Bank of New York publishes it at approximately 8:00 a.m. Eastern Time each US business day, based on transactions from the prior trading day.
It aggregates three repo segments:
- Tri-party repo excluding the Federal Reserve's own operations.
- General Collateral Finance (GCF) repo cleared by FICC.
- Bilateral Treasury repo cleared by FICC.
Combined daily volume is typically $1.8 to $2.5 trillion, making SOFR the deepest short-term rate benchmark in the world. It is a backward-looking rate derived from actual trades, not a survey.
SOFR went live on April 3, 2018, as the ARRC-recommended replacement for USD LIBOR. After USD LIBOR cessation on June 30, 2023, SOFR became the dominant USD floating benchmark.
The Intuition
A benchmark rate should reflect what borrowers actually pay, not what they say they would pay. Overnight Treasury repo is the largest and most liquid short-term dollar market. A bank or dealer with a Treasury bond can almost always borrow cash against it overnight, and hundreds of billions of those transactions clear every day.
Anchoring a benchmark to that market accomplishes two things. First, the rate is manipulation-resistant because it is a median of roughly one million individual trades. Second, the rate is nearly risk-free because each trade is collateralized by US Treasuries. That makes SOFR conceptually different from LIBOR, which embedded bank credit risk and ran on judgment in a thin term market.
How It Works
SOFR is calculated in four steps:
1. Collect eligible trades from three repo segments
(tri-party, GCF, bilateral Treasury, all Treasury-collateralized)
2. Remove specials (trades trading more than 25 bps below the GC rate)
3. Compute volume-weighted median of remaining trades
4. Round to two decimal places, publish at 8:00 a.m. ET next business day
Key terms and variants:
- Daily SOFR is the overnight rate. It is what SOFR publication means by default.
- SOFR Averages are 30, 90, and 180-day compounded daily SOFR, also published by the New York Fed.
- SOFR Index is a cumulative compounding factor useful for calculating interest accrual between any two dates.
- Term SOFR is a forward-looking 1M, 3M, 6M, or 12M rate published by CME Group, derived from SOFR futures. Use is limited by ARRC best-practice recommendations, mainly business loans and some securitizations.
Compounded SOFR in arrears is the default for most derivatives and floating-rate notes. The formula over a period of N business days, with daily SOFR = r_i and day count dc_i:
Compounded SOFR = [ (product over i=1..N of (1 + r_i * dc_i / 360)) - 1 ] * (360 / D)
where D is the total calendar days in the period. This formula preserves daily compounding while expressing the result as an annualized simple rate.
Worked Example
A five-business-day observation period with the following daily SOFR fixes and day-count factors:
Day 1: 5.30 percent, dc = 1
Day 2: 5.31 percent, dc = 1
Day 3: 5.28 percent, dc = 3 (Friday covers Sat + Sun)
Day 4: 5.32 percent, dc = 1
Day 5: 5.29 percent, dc = 1
Total calendar days D = 7
Compounding factors (1 + r * dc / 360):
1 + 0.0530 * 1 / 360 = 1.000147222
1 + 0.0531 * 1 / 360 = 1.000147500
1 + 0.0528 * 3 / 360 = 1.000440000
1 + 0.0532 * 1 / 360 = 1.000147778
1 + 0.0529 * 1 / 360 = 1.000146944
Product: roughly 1.001030. Subtract 1: 0.001030. Annualize over 7 calendar days:
Compounded SOFR = 0.001030 * (360 / 7) = 5.297 percent
The result approximates the arithmetic average because SOFR barely moved during the period. In fast-moving rate environments, the compounding effect diverges meaningfully, which is why the calculation is specified precisely in ISDA and loan documentation.
Common Mistakes
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Using simple averages when the contract specifies compounding. Most ISDA derivatives and ARRC-recommended loan conventions require daily compounding, not arithmetic averaging. A spreadsheet that averages daily SOFR fixes will systematically misprice payments, especially in volatile rate periods.
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Confusing Term SOFR with compounded SOFR. Term SOFR is known at the start of the period, making it operationally similar to LIBOR. Compounded SOFR in arrears is only known at the end of the period. Mixing the two without understanding the cash-flow timing difference causes reconciliation errors and hedging basis.
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Ignoring the lockout and lookback conventions. To give operations teams time to set payment amounts, many contracts use a two-business-day lookback or a lockout period for the final days. This means the observation period and the interest period do not perfectly align. Ignoring the convention produces day-count errors in settlement amounts.
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Treating SOFR as perfectly stable. SOFR is usually within a few basis points of the Fed's interest on reserve balances rate, but it can spike sharply when dealer balance sheets are strained. September 2019 saw SOFR print around 5.25 percent against an IORB of 2.2 percent, and intraday trades cleared near 10 percent. Modeling SOFR as flat near policy ignores tail risk.
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Forgetting SOFR Averages already exist. Many users rebuild compounded SOFR calculations manually when the New York Fed publishes official 30, 90, and 180-day compounded averages and the SOFR Index. Using the published values reduces operational errors and matches what indexing providers use.
Frequently Asked Questions
Q: What is SOFR in simple terms? SOFR is the interest rate at which financial institutions borrow cash overnight using US Treasury bonds as collateral. It is computed daily as the volume-weighted median of actual transactions, published by the Federal Reserve Bank of New York each business day, and serves as the reference rate for most new US dollar floating-rate contracts.
Q: How does the SOFR overnight rate affect investment decisions? SOFR is the benchmark for floating-rate bonds, loans, and interest rate swaps. Changes in SOFR flow through directly to floating coupon payments. Understanding whether a contract uses daily SOFR compounded in arrears or Term SOFR determines cash-flow predictability, which affects duration management and hedging needs.
Q: What is a real-world example of how SOFR compounding works? A five-day observation period with daily SOFR averaging around 5.30% and a Friday that counts for three calendar days (covering Saturday and Sunday) produces a compounded rate of approximately 5.297% for the week. The difference from a simple average is small in calm periods but diverges significantly when rates move fast during an easing or tightening cycle.
Q: How can investors use knowledge of SOFR mechanics? Always use the New York Fed's published 30, 90, and 180-day SOFR averages instead of rebuilding compounded calculations manually, they are official and reduce operational error. Check whether your floating-rate loan or bond uses Term SOFR (known at start) or compounded SOFR in arrears (known at end), as the choice affects timing of cash flows and hedge matching.
Q: How is SOFR different from the Fed funds rate? The Fed funds rate is the target rate for unsecured overnight interbank lending, set by FOMC policy. SOFR is a secured overnight repo rate based on Treasury collateral. SOFR normally trades within a few basis points of the interest on reserve balances rate, close to the Fed funds target, but it can spike sharply during funding stress when repo demand spikes or reserves drain.
Sources
- Federal Reserve Bank of New York. "Secured Overnight Financing Rate (SOFR)." https://www.newyorkfed.org/markets/reference-rates/sofr
- Federal Reserve. "Statistical Release H.15 Selected Interest Rates." https://www.federalreserve.gov/releases/h15/
- Alternative Reference Rates Committee. "SOFR Documentation and Resources." Federal Reserve Bank of New York. https://www.newyorkfed.org/arrc
- Securities Industry and Financial Markets Association. "SOFR Primer." https://www.sifma.org/resources/general/sofr-primer/
Disclaimer
This article is educational content only and is not financial advice. Nothing here is a recommendation to buy, sell, or hold any security. Consult a licensed advisor before making investment decisions.