• What is an invoice number?
• How to choose an invoice number format
• Invoice numbering examples
• Can you reset invoice numbers each year?
• What to do about gaps and duplicates
• Invoice numbering checklist

An invoice number is a unique reference that identifies one invoice in your records. A good system gives every invoice a different number, follows a predictable sequence, and stays consistent over time. You can use digits, letters, or both, provided each reference remains unique and your format meets local invoicing rules.

What is an invoice number?

The invoice number helps you and your customer locate the same transaction. It also connects the invoice to payment records, reminders, reports, and accounting entries.

It is different from a purchase order number. You create the invoice number, while your customer usually supplies the purchase order reference. Read our guide to P.O. numbers on invoices if a client asks for both.

Some jurisdictions require a unique identification number. UK invoice guidance lists one among the required invoice details, while EU VAT guidance calls for a unique sequential number on a full VAT invoice. Check the rules that apply where your business is registered before choosing a system. See the official UK invoice requirements and EU VAT invoicing guidance.

How to choose an invoice number format

Start with the simplest format that will still make sense as your business grows.

1. Pick a starting number that leaves room for future invoices.
2. Decide whether the year, client, or document type adds useful context.
3. Keep the sequence easy to continue without manual calculation.
4. Use the same format across your invoicing and bookkeeping workflow.
5. Keep the purchase order and project references in separate fields.

Your invoice number should be easy to read aloud and search. Avoid putting sensitive customer information inside it. Our step by step invoice guide covers the other information that belongs beside the number.

Invoice numbering examples

Choose one structure and document it. Changing formats repeatedly makes searching and reconciliation harder. Consistent numbering also supports the record keeping process described in our guide to organizing invoices and receipts online.

Can you reset invoice numbers each year?

You can start a new yearly series when local rules permit it, but the year must make each reference distinct. For example, 2026-0001 and 2027-0001 are different references. Resetting to a bare 0001 creates a duplicate and should be avoided.

If yearly reporting does not require a reset, one continuous sequence is usually simpler. Whichever method you choose, apply it consistently and retain the records from earlier series.

What to do about gaps and duplicates

Do not silently reuse a number after deleting or cancelling an invoice. Keep a record of the cancelled document and its reason, then continue with the next number. This preserves a clearer audit trail.

If two invoices receive the same number, correct the mistake promptly, tell the affected customer, and preserve the original record according to your local requirements. Our guide to common invoice mistakes explains other checks to complete before sending.

Invoice numbering checklist

Before issuing an invoice, confirm that:

• The number is unique
• The format matches your chosen sequence
• The purchase order number is stored separately
• The invoice date and customer are correct
• The payment terms are clear
• The document is saved in your records

Pair this checklist with the invoice payment terms cheat sheet so the invoice is easy to identify and easy to pay.