SONIA stands for the Sterling Overnight Index Average. It is the interest rate that banks and other large financial institutions pay to borrow sterling overnight, and it has become the backbone of the UK’s interest rate markets. You may never see it quoted on a mortgage offer, but it sits underneath a great deal of what moves gilts, sterling and the prices of loans and savings products.
What SONIA actually measures
Every business day, huge sums of money change hands overnight between banks, building societies, money market funds and other institutions. SONIA is the average interest rate paid on those unsecured overnight sterling deals. The Bank of England administers it and publishes the rate each London business day, reflecting transactions from the previous day.
Because it is based on real transactions rather than estimates, SONIA is considered robust and hard to manipulate. It is also usually very close to Bank Rate, the policy rate set by the Monetary Policy Committee, because the Bank’s operations keep short-term market rates anchored near that level.
Why it replaced LIBOR
For decades, the main benchmark for sterling borrowing was LIBOR, which was based on banks’ own estimates of what they would pay to borrow. After the rate-rigging scandals of the early 2010s, regulators decided that a benchmark built on estimates was too vulnerable. Sterling LIBOR was phased out from the end of 2021, and SONIA became the standard reference rate for new sterling contracts, from corporate loans to derivatives.
The United States made a similar shift to a rate called SOFR, and the euro area uses €STR. If you see these acronyms in market commentary, they are all doing the same broad job for their own currencies.
SONIA and Bank Rate
It helps to separate the two clearly. Bank Rate is the policy decision, announced at each MPC meeting. SONIA is the market outcome, the rate at which money actually changes hands overnight. When the MPC changes Bank Rate, SONIA follows almost immediately.
That close link is what makes SONIA so useful for traders. Markets can build contracts on future values of SONIA, and the prices of those contracts reveal what investors expect the Bank of England to do next.
How traders use it to read rate expectations
Two main tools are built on SONIA. SONIA futures trade on exchanges and let investors bet on the average rate over a future period. Overnight index swaps, or OIS, let two parties exchange a fixed rate for the floating SONIA rate over a set term. Our guide to overnight index swaps covers how those contracts work.
When commentators say markets are pricing a certain chance of a rate cut or hike at the next MPC meeting, they are usually reading those SONIA-based instruments. If the implied rate for the months ahead falls below the current Bank Rate, markets expect cuts. If it rises above, they expect hikes. Our explainer on how interest rate expectations move FX shows why that repricing matters so much for sterling.
Why it matters beyond the trading floor
Many business loans are now priced as SONIA plus a margin, so changes in the rate feed straight into companies’ borrowing costs. Fixed-rate mortgage pricing is linked to swap rates, which are themselves built on expectations for SONIA over the coming years. That is why mortgage rates can move before the Bank of England changes anything, simply because markets have changed their minds about the path ahead.
Savers feel it too. Money market funds and some savings products track short-term rates closely, so their returns rise and fall with SONIA.
What beginners often get wrong
A common mistake is assuming that because SONIA is overnight, it only matters for the very short term. In reality, expectations of future SONIA rates shape borrowing costs and asset prices years ahead. Another is confusing what markets expect with what will happen. SONIA pricing can swing sharply after an inflation release or a speech, and expectations are often wrong.
It is also worth remembering that the gap between UK and US rate expectations is one of the main drivers of GBP/USD. A shift in SONIA pricing after a UK data surprise can move cable more than the data itself appears to justify.
Bringing it together
SONIA is the average rate paid on overnight sterling borrowing, administered by the Bank of England and closely tied to Bank Rate. It replaced LIBOR as the UK’s main benchmark, and contracts built on it reveal what markets expect from the MPC. If you want to understand why gilts and sterling move on UK data days, learning to think in terms of SONIA expectations is a very good place to start.
If you would like to see how your understanding of rates and macro data compares with what professional traders need, our free trader assessment can show you where to focus.
