Concept Definition
What is an invoice number?
An invoice number is a unique sequential identifier assigned to each invoice by the issuer. It is a mandatory field on all VAT invoices and must be unique within the issuer's number series. Invoice numbers allow both parties to reference specific invoices in correspondence, payments, and tax filings.
What are the rules for invoice numbering?
Invoice numbering requirements under EU VAT rules (Article 226 of the VAT Directive):
- Must be sequential: No gaps in the sequence are permitted.
- Must be unique within a series: A supplier may maintain multiple numbered series (e.g., by customer or country), but each series must be internally sequential.
- Cannot be reused: Once assigned, an invoice number cannot be reused, even after a credit note.
- Series resets: Many businesses reset the sequence annually (e.g., INV-2025-00001).
Frequently Asked Questions
- Can invoice numbers contain letters?
- Yes. Invoice numbers can be alphanumeric. Common formats include a prefix (company initials or year) followed by a sequential numeric part. The key requirement is uniqueness and sequentiality within each series, not a specific format.
- What happens if an invoice number is duplicated?
- A duplicate invoice number is a compliance error. Tax authorities may flag duplicate numbers as evidence of duplicate invoicing or attempted double-claiming of input VAT. ERP and e-invoicing systems should enforce uniqueness at the point of invoice generation.