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SONIA: Definition & Meaning | Bondfish

What it means

SONIA stands for the Sterling Overnight Index Average. It is based on actual overnight transactions in the unsecured sterling money market, so it reflects real borrowing costs rather than estimates. The Bank of England administers and publishes SONIA at around 9:00 a.m. London time, covering the previous business day’s trades. As of 15 July 2026, SONIA stood at 3.73%.

Unlike SOFR, which is secured against Treasuries, SONIA is an unsecured rate — it captures lending without collateral, so it carries a small element of bank credit risk. Because a single overnight figure is volatile, lenders usually apply compounded SONIA in arrears over a period (for example, compounded three-month SONIA was around 3.75% in mid-July 2026).

SONIA is the UK’s replacement for sterling LIBOR, which was discontinued. It now underpins floating-rate sterling bonds, loans and the vast sterling interest-rate swap market.

Why it matters for bond investors

If you hold a sterling floating-rate note, its coupon is typically quoted as compounded SONIA plus a spread, so your income moves with SONIA. The rate also tracks the Bank of England’s policy stance closely, making it a useful gauge of short-term sterling funding costs and the direction of UK monetary policy.

Example

Floating coupon = compounded SONIA + spread
e.g. 3.73% (SONIA, 15 Jul 2026) + 0.90% = 4.63% for the period

Related terms

This definition is for general information only and is not investment advice. Bond investing involves risk, including possible loss of principal.