LIBOR-SONIA transition – compounded index to be published

08 July 2020

As a result of the consultation on the transition from LIBOR to SONIA, the Bank of England has confirmed that it will publish a compounded SONIA index from August. Adrian Joliffe of 2TIX LTD consultancy explains how this will work in practice.

It was recognised that for some organisations, calculating daily compounded SONIA from the raw data will be daunting. The compounded index will enable borrowers to verify interest payable on a SONIA-linked loan by replacing multiple compound interest calculations with a one-step formula. 

This proposal was fully supported by the NHF in their response to the consultation. What does it mean in practice?

The basic principle

Like any index, it is based on continuous compounding. From it you can work out the interest rate between any two dates and use this to calculate the interest payable.

In addition to the index, you will need the following information:

  • The amount of the loan.
  • The start and end dates of the interest period (and the number of days between the two).
  • The lag.
  • The margin.

In the following example, the interest period is 90 days, the loan amount is £1m and the 'lag' period is five London banking days. In your calculations, you should always use the relevant index figures for 'reference period', not the interest period. 

The reference period

Here, the reference period has the same 90 days as the corresponding interest period but ends five London banking days earlier, which are the five days of lag. 

Compounded SONIA rate = ((index from reference period end/index from reference period start) - 1) x (365/90)

Once you have calculated the Rate, interest payable is easy to calculate: 

Interest payable = £1,000,000 x (90/365) x (compounded SONIA rate + margin)

To make life even easier, Abovo-Consult and 2TIX LTD have jointly published a simple calculator on the 2TIX website for you to test your calculations. You will also find a SONIA glossary to help understand the new jargon and fully worked examples for calculating accrued interest and independently verifying interest payable. 

Changes announced to SONIA implementation timetable

In April, and in view of the coronavirus lockdown, the Working Group on Sterling Risk-Free Reference Rates recommended a change to the timeline for LIBOR transition:

  • By the end of Q3 2020, lenders should be in a position to offer non-LIBOR linked products to their customers.
  • After the end of Q3 2020, lenders, working with their borrowers, should include clear contractual arrangements in all new and re-financed LIBOR-referencing loan products to facilitate conversion ahead of end-2021, through pre-agreed conversion terms or an agreed process for renegotiation, to SONIA or other alternatives.
  • All new issuance of sterling LIBOR-referencing loan products that expire after the end of 2021 should cease by the end of Q1 2021.

Although these timings are later than originally, the final deadline of Q4 2021 has not changed.