What Is an Invoice Number? A Simple Guide for Businesses

An invoice number is the unique reference you give each invoice, such as INV-0042. You and your customer use it to find the invoice, match a payment to it, and refer to it in any question about the bill. In the UK and the EU, full VAT invoices must carry a sequential number from a series that identifies each invoice uniquely; simplified VAT invoices need fewer details. This page quotes those rules and labels each jurisdiction. It's general information, not legal or tax advice, so ask your accountant what applies to you.

Each source was opened on October 29, 2026. This page doesn't cover US rules on invoice numbers.

United Kingdom: every invoice

GOV.UK's guide Invoices: what they must include lists "a unique identification number" first among the details "Your invoice must include".

United Kingdom: VAT invoices

HMRC's VAT guide (VAT Notice 700) (last updated June 25, 2026), section 16.3.1, says a VAT invoice must show "a sequential number based on one or more series which uniquely identifies the document." Section 16.6.1 allows a simplified invoice if "the value of the supply you are making is £250 or less" and "your customer agrees", among other conditions. Its list of what "The simplified invoice must include" starts with "your name, address and VAT registration number" and doesn't include a sequential number.

HMRC's internal manual VATREC5010 (updated January 16, 2025) answers common questions about that rule:

• Letters and numbers: "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."
• Starting number: asked about starting at invoice number 1135 instead of 1, HMRC answers: "Yes, as long as the invoice series runs consecutively from that point."
• More than one series: "Yes, it is acceptable to operate more than one sequence at the same time."
• Canceled invoices: a break in the sequence is acceptable as long as the canceled or spoiled invoice is kept in your accounting records, or "you can provide an explanation for the break in sequence".

European Union: VAT invoices

Article 226 of Council Directive 2006/112/EC (the VAT Directive, consolidated text of April 14, 2025) lists, among the details required on VAT invoices, "a sequential number, based on one or more series, which uniquely identifies the invoice." Article 226 opens "Without prejudice to the particular provisions laid down in this Directive". Simplified invoices under Article 226b need fewer details, starting with "the date of issue" and "identification of the taxable person supplying the goods or services", and that list doesn't include a sequential number. EUR-Lex notes that a consolidated text "is meant purely as a documentation tool and has no legal effect," so check the rules of the country you invoice in with your accountant.

Formats you can use

The rules above don't set a format. Some common patterns:

• A plain number: 1001, 1002, 1003.
• A prefix and a number: INV-0042, INV-0043.
• The year and a number: 2026-0042, 2026-0043.
• A separate series for each business unit: A-0101 for one unit and B-0101 for another. HMRC's manual says: "Yes, it is acceptable to operate more than one sequence at the same time."

Whichever you choose, keep it unique, keep each series in order, and put the number where your customer can see it on the invoice.

Worked example: checking a series

The figures here are a sample. A studio uses series A for its UK clients. In March, its first invoice was A-0101 and its last was A-0137.

1. Numbers used in March: 137 − 101 + 1 = 37.
2. Invoices in the studio's records: 36 sent to clients + 1 canceled before it was sent = 37.
3. Check: 37 numbers used = 37 invoices on record, so the series has no unexplained gap.

The studio keeps the canceled invoice in its records. HMRC's manual says a break in sequence is acceptable as long as you retain the canceled or spoiled invoice, or can explain the break.

Check how any invoicing tool numbers invoices against the rules that apply to you, and ask your accountant.

Related Articles

• Invoice Compliance Requirements
• Invoice Numbering Practices
• What Is a VAT Invoice?
• What Is Accounts Receivable Management?
• What Is an Invoice?

Frequently Asked Questions

What is an invoice number?

It's the unique reference you give each invoice, such as INV-0042, so you and your customer can find it, match a payment to it and refer to it.

Do invoice numbers have to be sequential?

For full UK VAT invoices, HMRC's VAT Notice 700 requires "a sequential number based on one or more series which uniquely identifies the document." Article 226 of the EU VAT Directive has the same requirement for full EU VAT invoices. Simplified VAT invoices need fewer details: HMRC's list for them (Notice 700, section 16.6.1) and Article 226b of the EU directive don't include a sequential number. For other invoices and other countries, ask your accountant.

Can an invoice number include letters?

For full UK VAT invoices, HMRC's manual says: "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."

Can I run more than one invoice number series?

HMRC's manual says: "Yes, it is acceptable to operate more than one sequence at the same time." Article 226 of the EU VAT Directive also refers to "a sequential number, based on one or more series, which uniquely identifies the invoice."

What if I cancel an invoice?

HMRC's manual says a break in the sequence is acceptable as long as the canceled or spoiled invoice is kept in your accounting records, or the break can be explained. Ask your accountant how to handle it in your records.