This is general information, not tax or legal advice. Rules and rates change, so check GOV.UK or ask an accountant.
Invoice numbers look like a small detail, but a clear sequence makes your records trustworthy. If you are VAT registered, a VAT invoice needs a unique number, and HMRC expects the sequence to make sense. See GOV.UK on VAT records.
Choose a simple format
- Plain sequence: 0001, 0002, 0003.
- With a prefix: INV-0001. A prefix can separate invoices from credit notes (CN-0001).
- With the year: 2026-001, if you like to restart each year.
Avoid putting personal data in the number, and do not reuse a number.
Rules that keep it clean
- Every invoice has its own unique number.
- Numbers run in order, without deliberate gaps.
- Credit notes have their own sequence and refer to the original invoice.
- Drafts that were never issued can be removed, but note why.
If you spot a mistake
| Situation | Best fix |
|---|---|
| Wrong figure, not yet sent | Edit it before it goes out. |
| Created by mistake, nobody has seen it | Delete it. |
| Already sent to the customer | Issue a credit note, then a corrected invoice. |
| In a closed period | Credit note dated after the lock date. |
See credit notes explained and delete or correct an invoice.
Gaps in the sequence
A gap is not automatically a problem, but you should be able to explain it. A deleted draft, or a number used by a cancelled invoice, are common reasons. Keep a short note so the explanation is on file.
What goes on every invoice
See how to write an invoice and the complete invoicing guide.
How AvrioBooks helps
You enter the invoice number on each invoice, and the history panel records every change. Credit notes are numbered separately and linked to the original. Lock dates protect closed periods. See invoicing software.
Frequently asked questions
Does an invoice number have to be sequential?
It must be unique, and a clear sequence makes records easier to trust. A VAT invoice needs a unique number. Check GOV.UK for the rules.
What should I do if I sent an invoice with a mistake?
Issue a credit note, then send a corrected invoice, so the original stays on record.
Is a gap in my invoice numbers a problem?
Not automatically, but you should be able to explain it, for example a deleted draft.
Should credit notes have their own numbers?
Yes. A separate sequence with a reference to the original invoice keeps things clear.
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