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Sterling Overnight Interbank Average Rate (SONIA)


The Sterling Overnight Interbank Average Rate (SONIA) is a financial benchmark that represents the overnight interest rates paid by banks for unsecured transactions in the British sterling market. It is calculated as the weighted average of all overnight unsecured lending transactions in the London interbank market. SONIA is typically used as a reference rate for financial contracts and derivatives.


“Sterling Overnight Interbank Average Rate” would be phonetically transcribed as: /ˈstɜːrlɪŋ ˈoʊvərnaɪt ˌɪntərˈbæŋk ˈævrɪdʒ reɪt/ “SONIA” would be phonetically transcribed as: /ˈsoʊniə/

Key Takeaways

<ol><li>The Sterling Overnight Interbank Average Rate (SONIA) is a key benchmark interest rate for overnight unsecured transactions in the sterling market. It provides the basis for various financial products, such as derivatives and loans.</li><li>SONIA is administered by the Bank of England, which took over the responsibility in 2016, and is revised on a daily basis. The rate is calculated as the trimmed mean of interest rates paid on eligible deposit transactions, removing the top and bottom 25 percent and taking the average of the remaining.</li><li>SONIA has been reformed to meet international best practice for benchmark administration. Specifically, it has been recommended as the replacement for the London Interbank Offered Rate (LIBOR), which is being phased out due to concerns about manipulation and inadequate volume of underlying transactions.</li></ol>


The Sterling Overnight Interbank Average Rate (SONIA) is important because it is one of the main reference rates used in the British money markets to determine the cost of unsecured lending in the British banking system, making it crucial for a wide array of financial products like loans, mortgages, and derivatives. SONIA represents the average of the interest rates that banks are willing to lend to each other without collateral on an overnight basis. Following the LIBOR scandal, SONIA has been adopted as a more transparent and reliable benchmark, serving as a measure for setting other interest rates and as an indicator of the overall health and liquidity of the financial system.


The Sterling Overnight Interbank Average Rate (SONIA) serves an essential function in the financial markets of the United Kingdom. It’s a vital benchmark that reflects the cost of unsecured overnight borrowing in wholesale markets, in other words, it tells how much it costs banks to lend to each other on an overnight basis in the British pound sterling market. SONIA is deemed transparent and robust, making it integral to the efficient functioning of the UK money markets. Moreover, SONIA rates are used for a variety of financial purposes. It acts as a reference rate in the settlement of financial contracts like futures and options, including various lending transactions and derivatives products. The most significant application of SONIA is its usage as the basis for setting interest rates on most sterling-denominated loans and bonds, including mortgages and personal loans. Therefore, SONIA plays a crucial role in determining borrowing costs for businesses, households, and financial institutions alike.


1. Commercial Loans: One real-world example of the Sterling Overnight Interbank Average Rate (SONIA) in use would be commercial banks lending to businesses. Banks will lend to their clients and the loan’s interest rate is often tied to the SONIA, which fluctuates daily depending on the overnight interbank interest rate. Therefore, the interest rate a business pays their bank may increase or decrease according to the SONIA.2. Mortgages: Many mortgage providers in the UK tied their interest rates to the SONIA. For instance, if a homeowner takes out an adjustable-rate mortgage, their interest rate could go up or down based on the SONIA. This could mean potentially lower interest costs during periods of lower SONIA but increased costs when the rates go up.3. Financial Derivatives: SONIA is often used as a reference rate for financial derivative contracts such as Interest Rate Swaps. For example, in an interest rate swap, one party might agree to pay a variable interest rate based on the SONIA, while the other party pays a fixed rate. The variable rate party would benefit if SONIA decreases, while they would incur higher costs if SONIA increases. This allows financial parties to hedge their interest rate risk and potentially profit from interest rate movements.

Frequently Asked Questions(FAQ)

What is Sterling Overnight Interbank Average Rate (SONIA)?

SONIA is the average interest rate paid on overnight unsecured transactions in the British sterling market. It is used as the benchmark for short-term interest rates in the UK.

Who administers the SONIA?

SONIA is administered by the Bank of England.

How is SONIA calculated?

SONIA is calculated as a volume-weighted average of all overnight unsecured transactions with a minimum size of £25 million.

What are some common uses of SONIA?

SONIA is widely used in derivatives and loan contracts. Different types of financial institutions also use it in their day to day operations for a variety of purposes.

Why is SONIA important in finance?

As a benchmark interest rate, SONIA has a significant impact on the cost of borrowing for financial institutions. It can affect the interest rate on a variety of financial products, from loans to mortgages.

Is SONIA similar to LIBOR?

Yes, both SONIA and LIBOR are benchmark overnight rates, but SONIA is based on actual transactions, while LIBOR is based on estimates supplied by a panel of banks.

Has SONIA replaced LIBOR?

Since April 2018, SONIA has been positioned to replace LIBOR as the main benchmark rate due to concerns about the reliability and robustness of LIBOR.

What is the impact of SONIA on consumers?

SONIA can impact consumers indirectly. Changes in SONIA can lead to changes in the interest rates on products such as loans and mortgages which are linked to the rate.

How often is SONIA calculated?

SONIA is calculated and published by the Bank of England every London business day.

How can I find out the current SONIA rate?

The current SONIA rate is published on the Bank of England’s website.

Related Finance Terms

  • Interbank Rate: It’s the rate of interest charged on short-term loans made between banks.
  • Benchmark Rate: This is a standard against which the performance of a security, mutual fund or investment manager can be measured.
  • Overnight Index Swap (OIS): An interest rate swap involving the overnight rate being exchanged for a fixed interest rate.
  • Bank of England (BoE): The central bank of the United Kingdom which controls the issuance and supply of the British currency, the pound sterling.
  • LIBOR (London Interbank Offered Rate): This is the global benchmark interest rate that banks charge each other for short-term loans in the international interbank market.

Sources for More Information

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