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How to number invoices, and what happens if you skip one

An invoice number has to be unique and follow a sequence. It can have letters, restart each year, or run in several series. What UK, EU and Indian rules allow.

By Harish11 October 20264 min read

An invoice number has two jobs: it must be unique, and it must follow a sequence. Within those two rules you have a lot of freedom. It can contain letters, it can start at any number, it can include the year, and you can run more than one series at once. What you cannot do is give two invoices the same number, or leave gaps you cannot explain.

That is the whole answer for most people. The rest of this post is the detail behind it, which matters if you are registered for VAT or GST, or if you are about to choose a format you will live with for years.

What the rules actually say

The wording is close to identical across the places we checked. HMRC’s mirrors the EU’s almost word for word.

EU VAT Directive, Article 226: “a sequential number, based on one or more series, which uniquely identifies the invoice”.
HMRC, VAT Notice 700: “a sequential number based on one or more series which uniquely identifies the document”.
India, CGST rule 46: “a consecutive serial number not exceeding sixteen characters, in one or multiple series, containing alphabets or numerals or special characters”, where the special characters allowed are the hyphen and the slash, and the number must be “unique for a financial year”.

If you are not registered for VAT in the UK, GOV.UK only asks for “a unique identification number”, as one of the nine things every invoice needs. Sequential numbering is still the sensible choice, because it is what an accountant, a tax inspector and your future self all expect.

Three phrases in those rules carry most of the meaning.

“One or more series”. You may run separate sequences side by side. HMRC’s internal manual says so plainly: “it is acceptable to operate more than one sequence at the same time.” Germany’s tax guidance goes further and names the kinds of series it accepts: per year, month, week or day, and per branch or site, as long as each invoice can be traced back to its series.

“Uniquely identifies”. Two invoices with the same number is the one thing no authority allows. It breaks the only job the number has.

Letters are fine. HMRC: “The ‘invoice number’ can be numerical, or it can be a combination of numbers and letters, as long as it forms part of a unique and sequential series.” German guidance says the same, in German.

Gaps, cancelled invoices, and starting at 1135

People worry about gaps more than they need to, and less than they should about duplicates.

A gap usually means an invoice was cancelled, or a number was skipped by mistake. HMRC’s position is that this is acceptable “as long as you retain the cancelled or spoiled invoice in your accounting records, or you can provide an explanation for the break in sequence”. Germany’s guidance says outright that the numbers do not have to run without gaps. So:

Never delete an invoice you have sent. If it was wrong, keep it, mark it cancelled, and issue a new one with the next number. Editing a sent invoice and sending it again under the same number leaves two different documents that share one identity, which is the duplicate problem in disguise.
An unsent draft is different. If a number was never issued to anyone, reusing it is not a duplicate. Write down why the gap exists if you skip it.

You also do not have to start at 1. The same HMRC manual accepts a first invoice numbered 1135 “as long as the invoice series runs consecutively from that point”. Starting a new business at INV-0001 tells every client they are your first. Some people mind that and some do not. The rules do not.

Formats that work

A format should be easy to read aloud on the phone, sort correctly as text, and leave room to grow. Four that do:

Format	Example	Good for
Prefix and padded number	`INV-0042`	The default. Pad to four digits and you have 9,999 invoices before it looks odd.
Year and number	`2026-042`	Restarting each year, which is common in Europe and matches India’s financial year rule.
Client code and number	`ACME-007`	One series per client, when you bill a few clients often. Keep a separate list, because each series has to stay unique on its own.
Year, month and number	`2609-03`	Busy months. Short, and the date is readable at a glance.

And two to avoid:

Dates alone, such as 20260927. The second invoice on the same day has nowhere to go.
Unpadded numbers, such as INV-9 and INV-10. Any system that sorts by text puts 10 before 9.

In India, count the characters. Rule 46 caps the number at sixteen, and allows only letters, digits, hyphens and slashes. BLOOM/2026-27/0042 is eighteen, which breaks the rule. BLM/2627/0042 is thirteen and fine.

How our generator numbers them

The first invoice is INV-0001, unless you type something else. After that, each new invoice takes the number after the one you made most recently, and keeps its shape: it finds the last run of digits, adds one, and keeps the zero padding. So INV-0009 becomes INV-0010, 2026-099 becomes 2026-100, and ACME/12 becomes ACME/13. If your number has no digits at all, it adds -2 rather than replacing your scheme with ours.

You can type any number at any time, and the next invoice continues from it. That is how you start a new series for a new year: type 2027-001 on the first invoice in January.

Two limits, stated up front. It follows the most recent invoice, so if you run several series at once you will need to type the number when switching between them. And it does not yet warn you if you type a number you have already used, so check the list of your invoices beside the sheet before you send one by hand.