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.

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 our Database by 10am on the business day after it is first published. The database also includes historic data. 
The SONIA Key features and policies document provides more information on the benchmark. 

PDFSONIA Key features and policies

This page was last updated 25 April 2018
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