ICE Benchmark Administration



The ICE SONIA Indexes, published by ICE Benchmark Administration (IBA), are a set of SONIA Indexes, providing daily values that represent accrued compound SONIA interest, relative to the first day value of 100 on 23 April 2018 (the date the Bank of England first published reformed SONIA).

Developed to help address the key operational considerations of lenders and borrowers of SONIA-based loans, the index values are designed to provide a simple method for calculating compound SONIA interest between any two index dates, allowing parties to agree transparently on interest accruals.

Following the conclusion of an initial testing period, the ICE SONIA Indexes are being made available under licence from IBA for use in relation to financial contracts, alongside ICE Term SONIA Reference Rates (ICE TSRR), pursuant to IBA’s Master Licence Agreement.

Please read ICE Benchmark Administration’s (IBA) benchmark and other information notice and disclaimer here.

The following operational and economic features are included in the ICE SONIA Indexes;

Zero (0%) Floor Due to concern around potential negative UK interest rates in the future, many lenders would like to reference a SONIA Index with a minimum SONIA rate of 0% (“a floor”). If the daily SONIA value falls below 0%, the ICE SONIA index is calculated using 0% instead of the actual SONIA value.
Lookbacks To help manage cash flows and address operational issues with final repayments on loans before the end of a loan accrual period or the loan term, the ICE SONIA Indexes provide indexes calculated using a lag of either 2 or 5 business days.
Produced on weekends and holidays To facilitate accounting for loan accruals on reporting dates that are not business days, all ICE SONIA Indexes will provide index values for non-business days. Index values for non-business days will be published on the first following business day.

ICE SONIA Indexes are calculated daily and can be viewed (for information purposes only) here.

Parties wishing to use the ICE SONIA Indexes (including for the purposes of interest calculations for financial contracts) should obtain a usage licence from IBA. Prospective licensees should contact IBA’s licensing team for further information.


All ICE SONIA Indexes will use the same underlying calculation methodology for determining index values for business days. This is based upon the equation used by the Bank of England SONIA Index1


Compounded Indexi = The index for business day i, calculated and published on day i. All ICE SONIA Index values are rounded to 8 decimal places. This matches the rounding used by the Bank of England’s SONIA Index.
Compound Index1 = 100. All ICE SONIA Indexes each start with a value of 100. Day 1 for indexes without a lookback is 23 April 2018. For indexes with a lookback of N business days Day 1 is N business days later.
Compounded Indexi-1 = The index value calculated on business day i-1. While the published value of the index is always rounded to 8 decimal places, the underlying calculation uses previous day’s index values that have been rounded to 18 decimal places
SONIAi-1 = The SONIA rate for business day i-1, calculated and published on business day i
Weighting = The number of calendar days for which SONIAi-1 applies. This is equal to the number of calendar days between business day i-1 and business day i

ICE SONIA Index values for non-business days are linearly interpolated from the index values for the business days either side of the non-business days. For example, index values for Saturday and Sunday are typically interpolated from the index values for Friday and Monday.

Using the ICE SONIA Index to Calculate Interest

The interest accrued for a loan for a given period can be calculated using the index values on the start and end days of the loan:

For example, using the Index with No Floor and a 5 Day Lag, for a £100m loan from Tuesday 1 September 2020 to Wednesday 3 March 2021:

The following ICE SONIA Indexes are published each business day;

Floor Lookback Lookback Period (business days) Values Published for non-business days
None None Yes
Lag 2
0% None
Lag 2

The "SONIA" mark is used under licence from the Bank of England (the benchmark administrator of SONIA), and the use of such mark does not imply or express any approval or endorsement by the Bank of England. "Bank of England" and "SONIA" are registered trade marks of the Bank of England.