How to number your invoices

Invoice numbers let you, your customer and your accountant find and track every invoice. Many tax systems also expect numbers to be unique and sequential. Here are simple systems that work.

Rules that apply to every system

• Unique: never use the same number twice.
• Sequential: numbers should run in order without unexplained gaps.
• Never delete: if an invoice is wrong, keep it and issue a credit note.

Popular formats

• Simple sequence: 0001, 0002, 0003…
• Prefix + sequence: INV-0001. Different prefixes keep document types apart: QUO- for quotations, CN- for credit notes, DN- for delivery notes.
• Year + sequence: 2026-0001. Easy to sort; the sequence can restart each year if that is acceptable where you are.
• Customer code + sequence: ACME-0003. Useful with a few large customers, but keep an overall sequence too.

Leading zeros

Pad numbers with zeros (0001 rather than 1) so they sort correctly in spreadsheets and folders.

Keeping track

Keep a simple log of every number issued: date, customer, amount and status. In DocNova's invoice generator, the "Next document" button keeps your business details and moves to the next number automatically, for example INV-0101 to INV-0102.

What to do when you make a mistake

If you send an invoice with an error, don't edit it and send it again under the same number, and don't delete it. Issue a credit note that cancels it (fully or partly), then issue a new, correct invoice with the next number. Your records then show exactly what happened, which is what accountants and tax inspectors expect.

What if you skip a number?

Gaps happen, for example if you accidentally jump from 0041 to 0043. Don't panic. Keep a note in your log explaining the gap ("0042 not used: numbering error") so it can be explained if anyone asks. Avoid creating gaps deliberately.

Several people or several shops

If more than one person issues invoices, or you have branches, give each a prefix (for example LHR-0001 and KHI-0001) so numbers never collide, while each series stays sequential.

Separate series for other documents

Quotations, delivery notes, purchase orders and credit notes should each have their own numbering series. That way an invoice number always means an invoice, and nobody confuses a quote with a bill.