Small Business Accountants

What are Invoice numbers?

Invoice numbers are unique identifiers that UK businesses assign to every sales invoice they issue. HMRC requires invoice numbers on VAT invoices to be sequential and unique, so a VAT-registered business cannot skip or duplicate numbers without keeping records of why. Common numbering schemes include a plain sequential counter, a year-month-sequence format, or a customer-code-plus-sequence. Good numbering makes bookkeeping, reconciliation, and HMRC compliance checks easier.

By Jordan Macey MAAT Reviewed by Noor Fiaz Updated 16 Apr 2026 1

What are Invoice numbers? - Definition - GoForma Small Business | UK Accountants & Tax Advisors

Key takeaways

An invoice number is a unique reference that identifies a specific transaction, helping you track payment status and match records at year end.
VAT-registered businesses are legally required by the VAT Regulations 1995 to use unique, sequential invoice numbers on every VAT invoice they issue.
You can use any numbering format, such as INV-001, 2025-001, or a client-code prefix, provided each number is unique and the sequence never goes backwards.
Credit notes must carry their own separate sequential numbering series, distinct from your sales invoices, to avoid gaps or confusion in your records.
Non-VAT-registered businesses have no legal obligation to number invoices, but sequential numbering still simplifies bookkeeping and supports clean audit trails.

An invoice number is a unique number that is assigned to each invoice. This number is one of the most important elements of every invoice. Its role is to identify transactions, so it needs to be unique. Invoice number can contain only numbers or letters and numbers. It may contain date of issue, name of project or task.

Frequently asked questions

What is an invoice number?

An invoice number is a unique reference code you assign to each invoice you raise. It appears on the document so both you and your customer can identify and match a specific transaction. VAT invoices must carry a unique, sequential number under the VAT Regulations 1995. For non-VAT invoices there is no legal requirement, but consistent numbering keeps your records organised and simplifies any HMRC enquiry.

Do I legally have to put a number on my invoices?

If your business is VAT-registered, yes. The VAT Regulations 1995 require every VAT invoice to carry a unique sequential number. Missing or duplicate numbers can trigger penalties during a VAT inspection. If you are not VAT-registered, there is no legal obligation, but HMRC still expects you to keep orderly records under the general requirements for self-employed and company accounts.

What format should I use for invoice numbers?

HMRC does not prescribe a specific format. Common choices include a simple running sequence such as 001, 002, 003; a year prefix such as 2025-001; a client code prefix such as SMITH-001; or an INV prefix such as INV-0042. The only rules are that each number must be unique and the sequence must be consecutive. Avoid gaps unless you can document a voided invoice that caused them.

Can I include letters as well as numbers in an invoice number?

Yes. Invoice numbers can contain letters and numbers in any combination, and can also incorporate a date reference, project code, or client identifier. For example, 2025-04-SMITH-001 is perfectly acceptable. The key requirement for VAT invoices is that the full reference is unique across all invoices you issue and follows a sequential, non-repeating order.

What happens if I have a gap in my invoice numbers?

A gap in your sequence is not automatically a compliance problem, but it can raise questions during a VAT inspection if you cannot explain it. Common reasons include a voided or cancelled invoice. Keep a record of any cancelled invoice with its original number, the reason for cancellation, and the date. This demonstrates your sequence is intact and prevents HMRC from suspecting undeclared income.

Should credit notes have separate invoice numbers?

Yes. Credit notes should follow their own distinct sequential numbering series, separate from your sales invoices. For example, you might use CN-001, CN-002 for credit notes while invoices run INV-001, INV-002. Mixing them into the same sequence creates gaps in your invoice run and makes reconciliation harder. A separate credit note series keeps your records clean and is good accounting practice.

Do I need invoice numbers if I am not VAT-registered?

There is no legal requirement to number invoices if you are not VAT-registered, but it is strongly recommended. Sequential invoice numbers make it straightforward to chase unpaid bills, reconcile bank statements, and respond to any HMRC enquiry about your income records. Most accounting software assigns numbers automatically, so there is little reason not to use them from the start of your business.

Can I reset my invoice numbers at the start of a new year?

You can restart your sequence at the beginning of each financial or calendar year, provided the combination of the year reference and number remains unique. For example, using 2024-001 and then 2025-001 in the following year creates no duplication. Many businesses prefer a continuous sequence with no annual reset, which is simpler and avoids any risk of accidental duplication across years.

Need help with this for your business?

Book a free 20-minute call with one of our MAAT or ACCA qualified accountants. We will tell you honestly whether we can help.

203 5-star reviews

ACCA & AAT qualified

Set up in 24 hours