SONIA is the acronym for Sterling OverNight Index Average, the reference rate for overnight unsecured transactions in the Sterling market. It was launched in March 1997 by the Wholesale Markets Brokers' Association (WMBA), and is endorsed by the British Bankers Association (BBA).
Each London business day the SONIA fixing is calculated as the weighted average rate of all unsecured overnight sterling transactions brokered in London by WMBA members between 00:00 and 15.15 GMT in a minimum deal size of 25 million GBP with all counterparties listed under Section 43 of the Financial Services Act 1986.
The rate conventions are: annualised rate, act/365, four decimal places.