Accounting

What is Invoice Number?

An invoice number is a unique reference on each invoice, and for a VAT invoice HMRC requires it to be unique and part of a sequential series.

Example

A business issues invoices numbered INV-0001, INV-0002 and INV-0003 in date order. When INV-0002 is wrong, it issues a credit note and a new invoice rather than deleting it.

Key Dates

Assign a number when the invoice is created

How Invoice Number Works in Practice

An invoice number is the unique reference you print on every invoice. It lets you and your customer identify, match and refer to a specific invoice, and it gives you an orderly audit trail. For a VAT invoice, HMRC requires a unique identification number that follows on in sequence from the previous one.

There is no single required format. Many businesses use a simple running number, such as 1001, 1002, 1003, or a prefix plus a number, such as INV-0001. Some add the year, for example 2026-001, which makes it easy to see when an invoice was issued. Whatever scheme you choose, it should be easy to read, never repeat and run in order.

Gaps in the sequence can raise questions during a VAT inspection, because they suggest invoices may have been removed. If you make a mistake on an invoice, do not delete it and do not reuse the number. Instead, issue a credit note to cancel or correct the original and then issue a new invoice with the next available number. If you void an invoice that was never sent, keep a record of why the number was not used.

If you run more than one business, or issue invoices from more than one system, take care to keep sequences separate or consistent. Sequential numbering is easiest when all invoices come from one system, which assigns the next number automatically.

Step by Step

When you create an invoice, your system or your records assign the next number in the series. That number goes on the invoice, in your sales ledger and on any related credit note, so every document can be traced.

If you spot an error after sending, issue a credit note for the original amount, with its own number, then issue a corrected invoice with a new number. The record then shows exactly what happened and when.

Customers use your invoice number as the payment reference, which helps you match payments to invoices when they arrive in your bank account.

Practical Tips

Let your invoicing software assign numbers automatically
Use a separate prefix for credit notes, such as CN
Keep a note of any voided numbers and the reason
Do not restart numbering at the start of a new year unless the year is part of the number

Common Mistakes to Avoid

Reusing an invoice number, which makes invoices impossible to tell apart
Deleting an invoice instead of issuing a credit note, leaving a gap in the sequence
Starting a new sequence for each customer, so numbers are not unique across the business
Typing numbers by hand, which leads to duplicates and skipped numbers

Frequently Asked Questions

Does an invoice number have to be sequential?

For a VAT invoice, HMRC requires a unique number that follows on in sequence from the previous invoice. Even when you are not VAT registered, sequential numbering is good practice.

What should I do if I make a mistake on an invoice?

Issue a credit note to cancel or correct it, then issue a new invoice with the next number. Do not delete the original or reuse its number.

Can I use letters in invoice numbers?

Yes. Prefixes like INV or the year are fine as long as each number is unique and the series runs in order.

What if there is a gap in my invoice numbers?

Keep a record of the reason, such as a voided draft that was never sent. Unexplained gaps can raise questions in a VAT inspection.

Source: https://www.gov.uk/invoicing-and-taking-payment-from-customers

Related Terms

Invoice

An invoice is a document requesting payment for goods or services. VAT invoices must contain specific information for VAT-registered businesses.

Credit Note

A credit note is issued to reduce or cancel an invoice - used for returns, overcharges, or cancellations.

VAT (Value Added Tax)

VAT is a tax added to most goods and services sold in the UK. Businesses collect it from customers and pay it to HMRC, minus any VAT they've paid on business purchases.

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