What the rules say
GOV.UK says every invoice must have “a unique identification number”. For VAT invoices, HMRC is more specific: the invoice must show “a sequential number based on one or more series which uniquely identifies the document” (Notice 700/21, section 4.1).
In practice that means three things:
- Unique. No two invoices share a number, ever.
- Sequential. Each new invoice in a series follows on from the last, so gaps can be spotted.
- Letters are fine. HMRC’s guidance allows numbers, or a mix of letters and numbers, as long as they form a unique, sequential series (VATREC5010).
Sequential numbering matters for more than VAT. It shows that every sale has been recorded, makes it easy to find an invoice when a customer pays or queries it, and helps an accountant or HMRC follow your records.
Simple schemes that work
| Scheme | Example | Good for |
|---|---|---|
| A prefix and a running number | INV-0001, INV-0002 | Most small businesses |
| Year and a running number | 2026-001, 2026-002 | Seeing at a glance which year an invoice belongs to |
| A plain running number | 1001, 1002 | Keeping it as simple as possible |
A few tips:
- Pad with zeros (0001 rather than 1) so invoices sort in order in a folder or spreadsheet.
- You don’t have to start at 1. Starting at 1001 is fine, as long as you carry on in sequence from there.
- If you number by year, keep the year in every number. Restarting at 001 each year is fine only if the year is part of the number, so 2026-001 and 2027-001 are still different.
- Avoid numbering by customer (such as SMITH-1, JONES-1) unless each customer is a deliberate series of its own. It makes gaps hard to see.
Our invoice generator suggests the next number after the last one you downloaded on that device, if you ask it to remember your details. It keeps your prefix and zeros, so INV-0042 is followed by INV-0043.
Running more than one series
HMRC’s wording, “one or more series”, means you can run separate sequences side by side, for example one for each part of your business or each shop (VATREC5010). Each series must be unique and sequential in its own right. Give each one a different prefix, such as SHOP-0001 and WEB-0001, so numbers can never clash.
Mistakes, gaps and cancelled invoices
If you make a mistake on an invoice you’ve already sent, don’t delete it or reuse its number. Issue a credit note for it, then a new invoice with the next number.
If you spoil or cancel an invoice before sending it, keep it in your records marked as cancelled. HMRC’s guidance accepts a break in the sequence if you keep the cancelled invoice, or can explain the gap (VATREC5010).
Gaps without an explanation are what draw questions, because they can look like sales that weren’t recorded.