Last checked October 2026 · by the PaySorted team
The rules
| Not VAT registered | VAT registered | |
|---|---|---|
| Unique number | Required | Required |
| Sequential (in order) | Strongly recommended | Required |
| Letters as well as numbers | Fine | Fine |
| More than one series | Fine | Fine |
Gov.uk says every invoice needs "a unique identification number". VAT rules go further: a VAT invoice needs "a sequential number based on one or more series which uniquely identifies the document".
Good numbering formats
| Format | Example | Good for |
|---|---|---|
| Simple running number | 0001, 0002, 0003 | Most sole traders |
| Prefix + number | INV-0042 | Looks professional |
| Year + number | 2026-0042 | Easy to see the year |
| Separate series | DEP-0012 (deposits), INV-0042 | Different invoice types |
HMRC says numbers can include letters as long as they form a unique, sequential series, and you can run more than one series at the same time.
Can I start again at 1 each year?
Only if the number stays unique. "2026-001" and "2027-001" are different numbers because of the year, so starting a new series with a year prefix is generally fine. Starting again at plain "001" would repeat numbers you've already used — avoid that.
You can also start your first invoice at a higher number (so a new business doesn't look new), as long as it runs in order from there.
Gaps, mistakes and cancelled invoices
- Don't delete a wrong invoice — cancel (void) it and keep it in your records.
- If the customer has already received it, issue a credit note and then a new invoice.
- HMRC accepts a gap in the sequence if you keep the cancelled invoice, or can explain the break.
If you become VAT registered
If you're getting close to the £90,000 VAT threshold, get into the habit of sequential numbers now. You'll need them the day you register, and VAT invoices have a few extra must-haves — see what to include on an invoice.
PaySorted numbers every invoice for you automatically and in order, so you never duplicate a number or lose track of one — and your accountant sees the same sequence in Xero, QuickBooks, FreeAgent or Sage.
General information based on HMRC guidance (checked October 2026), not tax advice.
Common questions
Do invoice numbers have to be sequential in the UK?
For VAT invoices, yes. For non-VAT invoices, gov.uk only requires a unique number, but a sequential series is strongly recommended.
Can invoice numbers contain letters?
Yes. HMRC accepts a mix of letters and numbers as long as they form a unique, sequential series.
What if I skip an invoice number?
Keep a record of any cancelled or spoiled invoice, or be able to explain the gap. HMRC accepts gaps with an explanation.
Can I have different numbering for deposits?
Yes. You can run more than one invoice series at the same time, as long as each is unique and in order.