SONIA - An overview

What is SONIA?

SONIA (Sterling Over Night Indexed Average) is an overnight rate, set in arrears and based on actual transactions in overnight indexed swaps for unsecured transactions in the Sterling market. SONIA is a risk-free rate meaning no bank credit risk is included.

SONIA is expected to replace GBP LIBOR across global financial markets by the end of 2021.

How is SONIA Calculated?

Each London business day the SONIA fixing is calculated as the weighted average rate of all unsecured overnight sterling transactions brokered in London by Wholesale Markets Brokers’ Association (WMBA) members between 12am and 3.15pm London time in a minimum deal size of £25m.

Is SONIA a Term Rate?

SONIA is an overnight rate, not a term rate.

A term rate provides borrowers with a known interest rate for the period of borrowing and therefore provides up-front certainty of the amount of interest due at the end of the interest period. Some borrowers may find this helpful for their cashflow forecast.

SONIA is an overnight rate, based on actual market rates and reset on a daily basis in arrears; this removes any expectation of future events inherent in a forward-looking term rate.

There is some industry discussion about the possibility of creating a forward-looking “term SONIA” rate. However, the potential scope of where such a rate may be preferable, the methodology for its creation, and the timing of its introduction, all remain uncertain. The advice from the FCA is that firms should not wait for, or rely on, the development of any potential term SONIA rate.

Can I calculate a compounded rate for SONIA?

Our online 'calculator' shows you what the annualised compounded interest rate is for any defined period since the Bank of England started publishing the SONIA interest rate benchmark.