UK Late Payment Interest

Estimate late-payment interest, fixed compensation and totals for one UK business-to-business invoice in GBP. Submit invoice dates and an amount; receive one JSON result in the dataset and the key-value store's OUTPUT record.

The calculation uses simple interest at 8 percentage points above the applicable Bank of England reference rate. 1, 3 Fixed compensation is £40 for debts below £1,000, £70 from £1,000 to £9,999.99, and £100 from £10,000. 2

Not legal advice. The Actor estimates amounts; it does not determine eligibility or review contracts. GOV.UK says statutory interest cannot be claimed where a contract provides a different interest rate. 1

Input

{
  "amount": 2400,
  "invoiceDate": "2026-03-01",
  "termsDays": 30,
  "paidDate": "2026-06-15",
  "businessToBusiness": true
}

• amount: required JSON number, £0.01–£1,000,000,000, at most two decimal places.
• invoiceDate: required real calendar date in YYYY-MM-DD format, years 1000–9999.
• businessToBusiness: required; must explicitly be true.
• dueDate or termsDays: optional; supply at most one. termsDays adds 0–999 calendar days to invoiceDate. Omitting both uses this calculator's 30-day assumption; supply the actual due date where available.
• paidDate: optional payment date or projection end date. Defaults to today's date in Europe/London.

Unknown fields are rejected. Do not supply names or contact details.

Example output

Selected fields for the input above, run on 2 October 2026:

{
  "currency": "GBP",
  "dueDate": "2026-03-31",
  "paidDate": "2026-06-15",
  "asOfDate": "2026-10-02",
  "lateFrom": "2026-04-01",
  "daysLate": 76,
  "settled": true,
  "baseRate": 3.75,
  "annualRate": 11.75,
  "referenceDate": "2025-12-31",
  "estimate": false,
  "dailyInterest": 0.7726027397260274,
  "interest": 58.72,
  "compensation": 70,
  "claim": 128.72,
  "totalDue": 128.72,
  "total": null,
  "note": "Not legal advice. This is a calculation estimate; eligibility and contract terms need separate review."
}

The complete result also includes the invoice amount, dates, reference period, on-time flags and source URLs. The full example is in examples/output.json.

The Actor treats paidDate before today's London date as paid principal: totalDue contains interest plus compensation, and total is null. Today or a future date is treated as unpaid or projected: totalDue includes the principal. This is an input convention, so use dates accordingly. claim always means interest plus compensation; dailyInterest is the calculation rate, including for historical paid invoices.

On-time results have zero interest and compensation. Money totals are rounded to two decimal places after calculation; dailyInterest retains its precision.

Reference rates and limits

The reference rate is selected from the half-year containing the first day late and retained throughout that calculation. The January–June reference uses Bank Rate on the preceding 31 December; July–December uses Bank Rate on 30 June. 3 Bank Rate history supplies the underlying observations. 4

Rates must be updated each 1 January and 1 July. The maintainer must add the next half-year row to reference-rates.json using the corresponding reference date, then run the tests. Updates are manual; the Actor does not fetch live rates. 3, 4

The bundled table covers first-late dates from 1 January 2020 through 31 December 2026. Earlier late debts are rejected. Later first-late dates reuse the final row and return estimate: true with a warning. A payment date beyond table coverage alone does not change the reference rate or set that flag.

The Actor accepts one invoice per run. Partial payments, multiple invoices, contract-specific interest and additional recovery costs are outside its calculation model.

Local use

With Node.js 22 or later, run node cli.js examples/input.json for a dependency-free calculation, or npm test to run the calculator, Actor and timezone regression checks. The example's asOfDate changes with the run date. Deployment uses the included Actor metadata and Dockerfile.

Sources

1. GOV.UK: Interest on late commercial payments. Accessed 2 October 2026.
2. GOV.UK: Claim debt recovery costs. Accessed 2 October 2026.
3. GOV.UK: A users guide to the recast Late Payment Directive, pages 7–8. Accessed 2 October 2026.
4. Bank of England: Bank Rate history. Accessed 2 October 2026.