SONIA-linked business loans: understand the benchmark before approving the interest bill

SONIA is the Bank of England's sterling overnight benchmark and is widely used in floating-rate corporate loans and derivatives. A SONIA-linked facility usually charges compounded SONIA for the interest period plus a lender margin, so the borrower needs to understand both the benchmark and the contract's calculation convention.

SONIA measures overnight sterling wholesale funding

The Bank of England defines SONIA as a measure of the rate paid on eligible overnight sterling wholesale transactions where credit, liquidity and other risks are minimal. It administers and publishes the benchmark every London business day.

The company does not borrow overnight from the Bank of England. The loan contract simply uses the published benchmark as one component of the interest rate paid to the lender.

Corporate loans commonly use compounded SONIA over the interest period

A three-month interest period can compound daily SONIA observations rather than using one rate fixed at the start. The Bank of England's SONIA Compounded Index can simplify calculation between specified dates.

Because the final benchmark is built over the period, the exact interest amount can become fully known only near the payment date. Treasury should therefore forecast rather than wait for the final notice.

The lender adds a credit margin

The borrower normally pays compounded SONIA plus a lender margin reflecting credit risk and facility terms. A quoted margin of 2.25 percent does not mean the all-in rate is 2.25 percent.

Keep margin and benchmark separate in debt reporting so directors can distinguish changes in market rates from changes in the company's own credit pricing.

Read lookback, observation-shift and floor provisions

Loan agreements can use a lookback period so the final interest amount is known a few days before payment. They can also contain zero floors or other benchmark conventions that affect the amount payable.

Do not recreate the interest by averaging published daily SONIA casually. Use the exact method in the facility agreement and compare it with the lender's calculation.

Forecast SONIA-linked interest with scenarios

Treasury can use market expectations for budgeting but should also stress the debt cost above and below the central forecast. Floating interest affects cash before principal changes.

A business with multiple SONIA-linked facilities should aggregate exposure. One loan can look manageable while the combined benchmark sensitivity across the group is material.

Reconcile every interest notice to principal, benchmark and margin

Check the opening principal, repayments during the period, compounded benchmark, margin, day-count basis and payment date. Store the lender's interest notice with the debt schedule.

If the company's calculation differs, investigate promptly. Floating-rate debt should not become an unexplained bank debit simply because the formula is technical.

Worked example: a £12 million facility carries compounded SONIA plus 2.0 percent. If the compounded benchmark for the period is 3.8 percent, the approximate annualised all-in rate is 5.8 percent before fees and any floor effects. Finance should show both components rather than reporting one blended percentage with no explanation.

Use the Bank of England's published index or the lender's contractually specified source consistently. Mixing one bank's spreadsheet with a different observation period can create apparent differences that are only methodology differences rather than billing errors.

When refinancing, compare margin as well as SONIA convention. Two facilities can both say "SONIA plus 2 percent" but produce slightly different cash interest because one uses different lookback, rounding, day-count or floor provisions.

Use contractual interest-period dates rather than calendar quarters when forecasting. A facility can run from 17 March to 17 June, and the compounded benchmark should follow those dates and the agreement's business-day conventions. Treasury forecasts built only by month can otherwise misstate the exact payment date and accrued interest at quarter end.

Check benchmark fallback language. Loan documents should explain what happens if SONIA is unavailable or the benchmark methodology changes. This is rarely important in normal conditions, but a fallback becomes critical precisely when markets are stressed and parties do not want to negotiate emergency amendments.

For facilities with margin ratchets, combine the benchmark forecast with expected leverage. A company can face higher interest from both rising SONIA and a wider margin after weaker trading. Stress testing should show the combined effect rather than one variable at a time.

At month-end, accrue interest using the contract's best available estimate even if the final compounded rate is not yet known. The accounting estimate can then be trued up when the lender notice arrives. This prevents floating-rate expense from disappearing from management accounts until payment day.

Monitor reference-rate floors. If the loan has a zero floor or a higher contractual floor, the borrower might not receive the full benefit of very low SONIA. Forecast models should therefore use the actual loan formula rather than assuming all benchmark reductions pass through one-for-one.