How to number your invoices

The two rules that matter

Invoice numbers must be unique: two invoices must never share a number. And they should be sequential: each new number follows the previous one, so it is obvious if one is missing. EU VAT rules, for example, require “a sequential number, based on one or more series, which uniquely identifies the invoice”.

Beyond that you have a lot of freedom. You can use prefixes, include the year, or run separate series, as long as each series is consistent and every number is used only once.

Popular numbering formats

• Simple sequence: 0001, 0002, 0003
• Prefix and sequence: INV-0001, INV-0002
• Year and sequence: 2026-001, 2026-002 (restart each year)
• Year, month and sequence: 2026-10-001
• Client code: ACME-001, ACME-002 (a separate series per client)
• Project code: WEB-2026-01 for project-based work

Choosing the right format

If you send a handful of invoices a month, a simple prefix and sequence is all you need. Including the year makes it easy to see when an invoice was issued and lets you start again at 001 each January, which is allowed in many countries as long as the year makes each number unique.

Per-client series help when you work with a few large clients, but make sure every number is still unique across your whole business, which is why client codes are usually combined with a prefix or year.

Start with enough leading zeros for your volume (0001 rather than 1) so the numbers sort correctly in spreadsheets and folders.

Credit notes, quotes and other documents

Use a separate series for other document types so they never clash with invoices: CN-001 for credit notes, Q-001 for quotes, PO-001 for purchase orders, R-001 for receipts. A credit note should also reference the number of the invoice it corrects.

Never reuse or edit the number of an invoice you have already sent. If an invoice is wrong, issue a credit note and a new invoice rather than changing the original.

What to do about gaps and mistakes

A gap in the sequence, for example if you created an invoice and deleted it before sending, isn’t a disaster, but you should be able to explain it. Keep a note of cancelled numbers rather than reusing them. Tax authorities look for unexplained gaps because they can indicate missing sales.

If two invoices accidentally share a number, contact the client, cancel one with a credit note and reissue it with the next free number.

Numbering in practice: an example

A freelancer who restarts each year and keeps credit notes in their own series might have a log like this:

2026-001 12 Jan Acme Ltd £1,200.00 2026-002 30 Jan Northwind Ltd £640.00 2026-003 14 Feb Acme Ltd £1,200.00 CN-2026-01 20 Feb Credit for 2026-003 –£200.00 2026-004 03 Mar Lumen Studio £980.00

Switching to a new system

If you move from one tool to another, carry on with the next number in your existing sequence rather than starting again at 1, or start a new series with a different prefix or year so no number can be repeated. Note the date of the switch in your records.

Several businesses, branches or currencies

Each legal entity needs its own sequence, because each keeps its own books. Within one business you can run separate series for branches, sales channels or currencies, for example LON-001 and NYC-001, as long as each series is consistent and no number appears in two series.

Some countries now route invoices through government e-invoicing systems that register or validate each invoice. If that applies to you, follow the numbering rules of that system and your accounting software.

Should your numbers reveal how many clients you have?

A brand-new business sending invoice 0001 isn’t a problem, but some freelancers prefer not to show it. Starting at a higher number, or using a year-and-month format such as 2026-10-01, is generally acceptable as long as the sequence is unique and consistent from then on.