Late payment interest calculator for UK trades: what a late business customer owes you

If a business customer pays you late, the law lets you add interest and a fixed fee to the bill. The calculator below works out both. Put in what you are owed, when it was due and when it was paid (or today's date if it still has not been), and it does the rest.

Free tool

Late payment interest calculator

Amount owed (£, the invoice total including VAT) Did you agree a payment date?Yes, a date was agreedNo date was agreed Agreed payment dateIf no date was agreed, enter the later of the day you finished the work and the day the customer got your invoice; we add 30 days. Date paid (or today, if still unpaid)

Works in your browser. Nothing you type leaves this device.

How the sum works

There are three parts, and the calculator shows each one.

The rate. Statutory interest is 8% a year plus the Bank of England's Bank Rate. The catch is which Bank Rate. The rate order says you use the Bank Rate in force on a fixed reference day: 31 December for interest that starts between 1 January and 30 June, and 30 June for interest that starts between 1 July and 31 December. The rate is then fixed for that debt. It does not move if the Bank changes its rate while you wait.

Bank Rate was 3.75% on 31 December 2025 and still 3.75% on 30 June 2026, so any interest that started running in 2026 is charged at 11.75% a year. A debt whose interest started in the first half of 2025 uses the 31 December 2024 rate of 4.75%, so 12.75%. Many free calculators online just ask you to type in "the current base rate", which gives the wrong answer for older debts.

The days. Interest starts the day after the payment was due. If you agreed a payment date, that is the date. If you never agreed one, the Act gives the customer 30 days, counted from the later of the day you finished the job and the day they got your invoice. Choose "No date was agreed" in the calculator and it adds the 30 days for you.

The fixed fee. On top of the interest you can claim a fixed sum towards the cost of chasing: £40 if the debt is under £1,000, £70 from £1,000 to £9,999.99, and £100 for £10,000 or more. You can claim it once per invoice.

Worked examples

We ran four typical trade invoices through the calculator at 11.75%:

Two things stand out. On small invoices the fixed fee is worth more than the interest, so it is the part to claim even if the customer is only a few weeks late. And the interest on a big invoice builds by the day: £12,000 at 11.75% adds £3.86 every day it stays unpaid. Our longer guide on what late payment interest a builder can actually claim has more sums and the cases where you cannot claim.

Who you can charge, and who you cannot

The Act covers business-to-business debts. Main contractors, developers, letting agents, landlords with a company, shops and offices all count. A householder having a rewire does not. You cannot add statutory interest to a consumer's bill unless your contract with them said you would charge interest, and even then it has to be a fair term.

You also cannot use it if your contract already sets out a "substantial remedy" for late payment. If your terms say 2% a month, that is what applies, not the Act.

How to claim it

Send a new invoice for the interest and the fixed fee, as GOV.UK suggests, rather than changing the original. Say what it is for, the rate, the number of days and the dates. Our invoice maker for trades puts the late payment line on every invoice, so customers know from day one.

Then keep chasing. Most customers pay the original bill once they see interest on paper; whether you waive the interest at that point is a business call, not a legal one. The three chasing letters show when to mention it: the second letter warns, the third claims.

This is general information, not legal advice. For large debts, or if the customer disputes the work, talk to a solicitor or your trade body's legal helpline before you go further.