Further details about SONIA data

The Bank of England is the administrator for SONIA, the Sterling Overnight Index Average.


The definition of SONIA has two elements:

(i) Statement of underlying interest

SONIA is a measure of the rate at which interest is paid on sterling short-term wholesale funds in circumstances where credit, liquidity and other risks are minimal.

(ii) Statement of methodology

On each London business day, SONIA is measured as the trimmed mean, rounded to four decimal places, of interest rates paid on eligible sterling denominated deposit transactions. 

The trimmed mean is calculated as the volume-weighted mean rate, based on the central 50 per cent of the volume-weighted distribution of rates. 

Eligible transactions are:

  • reported to the Bank’s Sterling Money Market daily data collection, in accordance with the effective version of the ‘Reporting Instructions for Form SMMD’; 
  • unsecured and of one business day maturity;
  • executed between 00:00 hours and 18:00 hours UK time and settled that same-day; and
  • greater than or equal to £25 million in value.

The statement of the underlying interest is intended to be an enduring statement of the economic concept that SONIA seeks to measure. The statement of the methodology describes how the specified underlying interest is currently to be measured.


The Sterling Overnight Index Average (SONIA) is a widely used interest rate benchmark and the reference rate for sterling Overnight Indexed Swaps (OIS).

SONIA was introduced in March 1997. The Bank of England became its administrator in April 2016, and we introduced a series of reforms of the SONIA benchmark in April 2018.

The daily data series corresponding to SONIA data are:

IUDSOIA Sterling Overnight Index Average

IUDZLT2 Total Nominal Value

IUDZLS6 Percentile Ten

IUDZLS7 Percentile Twenty Five

IUDZLS8 Percentile Seventy Five

IUDZLS9 Percentile Ninety

SONIA for the previous London business day is published by authorised distributors at 9am. 

It is made freely available, via our Database by 10am on the business day after it is first published. The database also includes historic data.

The SONIA interest rate benchmark page and Key features and policies page provide more information on the benchmark. 

SONIA key features and policies

Further details about interbank sterling market

Interbank sterling market

The unsecured interbank deposit market began to develop in the 1960s as the wholesale market in which banks and others lend and borrow money for predetermined periods ranging from overnight to one year, to accommodate short-term liquidity needs or for the lending on of surplus funds. This market was given further impetus by the deregulatory reforms of the early 1970s, the abolition of exchange control at the end of the 1970s and the final lifting of the retail banks lending ceilings. The 1980s saw a surge in intermediation as the newly liberalised banking system greatly extended the range of financing facilities provided to its industrial and commercial customers.

Since 1985, the rates shown are representative rates and are the mean of the bid and offer rates obtained from a minimum of three brokers at 8.30 am; for earlier periods the figures are described as the mean of the lowest bid and highest offer rates over the day.

The figures shown for the one month rate for 1970-1977 inclusive are for the last Friday of each period. The figures shown for the three months rate for 1970 and 1971 are for the last Friday of the month; from 1972 - 1977 inclusive, the figures are the average of Fridays in each period.

These are quoted (nominal) rates and are the mean of the bid and offer rates from three or more brokers at 8.30am.

No data available from 5 December 2017 because the Bank has been unable to source sufficient representative data to allow it to calculate an average rate.  This series has been discontinued.

This page was last updated 16 April 2020

Give your feedback

Was this page useful?
Add your details...