How to Number Invoices: Formats and Examples

Facts last checked October 24, 2026

An invoice number is a label that lets you and your client point at one specific invoice. It looks like a small detail, but a sensible numbering system saves time every time someone asks “which invoice do you mean?”, and some tax authorities expect it.

What the rules say

Official guidance is short and consistent on the main point: every invoice needs a number that identifies it uniquely.

UK: GOV.UK lists “a unique identification number” among the things an invoice must include. The page does not say the numbers have to run in order. See GOV.UK, invoices: what they must include.
EU: the VAT Directive asks for a sequential number, based on one or more series, that uniquely identifies the invoice. Using several series is allowed as long as each invoice can still be told apart. We reached the text through a UK-hosted copy as it stood in 2020, so confirm against the current law: Article 226.
Elsewhere: requirements vary. If you are registered for a sales tax, ask your tax authority or an accountant whether they expect an unbroken sequence.

Whatever your country, the practical rule is the same: never give two invoices the same number.

Common formats

Format	Example	Good for
Prefix and running number	`INV-0001`	Most freelancers and small businesses
Year and running number	`2026-001`	People who like to see the year at a glance
Year, month and running number	`2026-10-004`	Businesses issuing many invoices a month
Client code and running number	`ACME-007`	Businesses with a few regular clients
Plain running number	`1042`	Simple, but easy to confuse with order numbers

Pick one format and keep it. Changing the pattern halfway through the year makes your records harder to follow.

Use leading zeros

INV-0009 sorts before INV-0010 in a list. INV-9 sorts after INV-10 in many systems, because the characters are compared one by one. Leading zeros keep the order right. Choose enough digits for your volume: four digits (0001 to 9999) is plenty for most small businesses.

How the next number is chosen

The simplest method is to add one to the last number. In our invoice generator, the number is increased in the last run of digits and the padding is kept:

Last invoice	Next invoice
`INV-0009`	`INV-0010`
`INV-099`	`INV-100`
`2026-009`	`2026-010`
`INV-9`	`INV-10`

If you tick the option to remember your details on your own device, the tool suggests the next number for your following invoice.

Should you start at 1?

Starting at 0001 is fine. Some people start at a higher number, such as 1001, so a new business looks more established. There is nothing wrong with that, but do not invent a history: the numbers should reflect the invoices you have actually issued.

What to do when you make a mistake

Do not edit an invoice that has already been sent, and do not delete its number. Instead:

Issue a credit note that cancels or reduces the original, and refer to the original invoice number on it.
Issue a new invoice with the next number in your sequence if the work still needs billing.
Keep the cancelled invoice in your records.

Many businesses give credit notes their own series, for example CN-0001. If you use a separate series, make sure the numbers within it are still unique.

If you do not issue many invoices

You may not need a complicated system. If you issue a few invoices a year, INV-0001, INV-0002 and so on will serve you well for years. If you issue dozens a month, a year-and-month format makes finding an invoice much easier.

Common mistakes

Reusing a number after deleting a draft that was already sent.
Different formats on different invoices, such as INV-12 and 2026/013.
Numbers that depend on the client, when you share one record for tax.
Losing track of voided numbers. Keep a simple log, such as a spreadsheet.

Sources and notes

This guide is general information, not tax or legal advice. Rules differ by country, so check with your tax authority or an accountant. If you are unsure what your invoice needs besides a number, see what to include on an invoice.

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