Free tool
Invoice number generator
Pick a format that HMRC and your bookkeeper will be happy with, then copy your next invoice numbers. Sequential, unique and no gaps.
Your format
Include the year
e.g. INV-2026-0001
Include the month
Useful if you invoice heavily and file monthly.
Your next ten numbers
Tap any number to copy it. Use them in order and never reuse one.
The rules on invoice numbers in the UK
Every invoice you issue must have a unique identifying number that follows on from the last one. That is the whole legal requirement — there is no prescribed format. The point is that HMRC, or anyone auditing your books, can follow the run from start to finish without gaps.
Formats that work
- INV-0001 — simplest option, fine for a sole trader doing a few invoices a month.
- INV-2026-0014 — the year up front makes filing and year-end far easier.
- INV-2026-03-0007 — year and month, worth it if you invoice daily.
- INV-SMITH-0003 — a customer code helps on repeat commercial work, as long as the number itself still increases.
Zero padding (0001 rather than 1) keeps numbers sorting correctly in folders, spreadsheets and email subject lines.
What to avoid
- Reusing a number after cancelling an invoice — issue a credit note instead.
- Starting at 1 if you would rather not advertise it — starting at 1001 is fine.
- Random numbers or dates alone, which make gaps impossible to prove.
- Changing format mid-year without keeping the sequence continuous.
- Running two sequences from two apps and duplicating numbers.
Quotes and estimates need their own sequence
Keep quotes on QUO- and estimates on EST- so nothing is mistaken for a request for payment. When a quote is accepted, issue a fresh invoice number and reference the quote number on it. Build the documents with the free quote generator and the free invoice generator.
Stop tracking numbers by hand
Telaro assigns the next number automatically, so you can never duplicate or skip one — plus customers, VAT, CIS and who hasn't paid. Free, with 10 invoices a month.