Glossary term
Sterling Overnight Index Average (SONIA)
SONIA is the Bank of England’s sterling overnight interest-rate benchmark based on eligible unsecured overnight transactions.
Updated
Read time
What Is SONIA?
The Sterling Overnight Index Average (SONIA) is the Bank of England's sterling overnight interest-rate benchmark based on eligible unsecured overnight transactions. It is a core reference rate for sterling money markets and many contracts that moved away from sterling LIBOR.
SONIA is an overnight rate. It is often used in compounded form when applied to loans, bonds, or derivatives over a longer interest period.
Key Takeaways
- SONIA is administered and published by the Bank of England.
- It reflects eligible sterling unsecured overnight transactions.
- It became the main sterling risk-free reference rate after LIBOR transition.
- Contracts using SONIA need compounding, timing, day-count, and fallback conventions.
- SONIA is a base benchmark, not the full borrowing cost paid by a company or household.
How SONIA Works
SONIA is calculated from overnight sterling transactions in the wholesale market. Because it is transaction-based, it is designed to be anchored in actual market activity. The rate is published for a past overnight period rather than estimated at the start of a future term.
That backward-looking feature matters. A compounded SONIA interest amount for a loan or bond may be calculated during or near the end of the interest period, depending on the contract's observation convention.
Where SONIA Appears
Market | How SONIA Is Used |
|---|---|
Derivatives | Reference rate for sterling overnight indexed swaps. |
Floating-rate notes | Base rate for coupon calculations. |
Loans | Benchmark rate compounded over an interest period. |
Valuation | Discounting and curve-building in sterling markets. |
SONIA Versus Sterling LIBOR
SONIA differs from LIBOR in several important ways. LIBOR was an interbank offered rate available in multiple forward-looking tenors and included bank credit and term components. SONIA is overnight and nearly risk-free by comparison. That makes it more robust for benchmark purposes but different economically.
When a contract moves from LIBOR to SONIA, a spread adjustment or new pricing convention may be needed so the economics are not distorted. The details depend on the contract, market, and fallback language.
How to Read It
SONIA is best read as a sterling base-rate benchmark. It does not include a borrower's credit risk, bank margin, term liquidity premium, or product fees. A corporate loan priced at compounded SONIA plus a spread can move with overnight sterling money-market conditions, but the spread and conventions determine the final borrower cost.
Investors should check whether a security uses daily SONIA, compounded SONIA, an index version, or another convention. Small timing differences can matter across large notional amounts.
Contract Details That Matter
SONIA-based contracts often specify whether the rate is compounded in arrears, whether there is an observation shift, how many business days of lag apply, and how non-business days are handled. These mechanics affect payment certainty and operational timing.
Borrowers and investors should also watch basis risk. A hedge using one SONIA convention may not perfectly offset a loan or bond using another. The benchmark name is only the start; the convention turns the overnight rate into a payable amount.
SONIA also helps explain why benchmark transition was more than a name change. Moving from a term credit-sensitive rate to an overnight transaction-based rate changes systems, documents, hedge accounting, borrower communication, and cash-flow calculation processes.
For investors in floating-rate notes, SONIA affects income timing and reinvestment expectations. For borrowers, it affects interest expense and hedge design. The same benchmark can therefore be a portfolio-income input and a corporate treasury risk factor.
That practical link between benchmark design and cash flow is why SONIA appears in both market-risk and treasury-management discussions.
The Bottom Line
SONIA is the Bank of England's sterling overnight benchmark rate. It replaced much of sterling LIBOR's role, but users still need to understand compounding, spreads, reset mechanics, and fallback language.