Late payment interest calculator (UK)

Base rate checked on 15 October 2026: 3.75% on the Bank of England website (unchanged since 18 December 2025; next decision 5 November 2026). The field is editable. For an older invoice, the law uses the base rate in force on 30 June if interest starts to run between 1 July and 31 December, or on 31 December if it starts between 1 January and 30 June. Interest starts the day after the due date, so a 2026 invoice due in the second half of the year uses the 30 June 2026 rate (3.75%).

Days overdue
Annual interest rate (8% + base rate)
Interest per day
Interest to date
Fixed compensation
Total you can claim (on top of the invoice)
Guidance, not legal advice. This calculator gives an estimate based on the Late Payment of Commercial Debts (Interest) Act 1998 and runs in your browser. It doesn't know your contract, and the Act doesn't apply if you agreed a different, substantial late payment remedy or the customer is a consumer. If the sum is large or disputed, get advice from a solicitor, Citizens Advice or your trade body.

Free late payment chaser letter

Copy this letter or email into your own message. It fills in the figures from the calculator above (change the amount, dates or rate and it updates until you edit the text), and you complete the square-bracket parts. Send it once a final reminder is overdue; keep a copy and proof of sending.

Estimate only, not legal advice. For business-to-business debts under the Late Payment of Commercial Debts (Interest) Act 1998.

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