Invoice Numbering

Every VAT invoice issued in Poland must carry a number that uniquely identifies it. This article explains the legal rules behind invoice numbering and how the invoice number differs from the KSeF reference number assigned once a document is accepted.

Legal Requirement and Continuity

Under Art. 106e(1)(2) of the Polish VAT Act, every invoice must include a sequential number that unambiguously identifies it within one or more numbering series. The core rule behind this requirement is continuous numbering (numeracja ciągła): within a given series, numbers must follow one another without gaps.

• A gap in a series can raise questions during a tax audit, since it may suggest an invoice was issued and then removed or never recorded
• Once issued, an invoice is never deleted or renumbered to "fix" a mistake -- errors are corrected with a corrective invoice instead
• Continuity is evaluated per series, not across a business as a whole

Numbering Series and Formats

A business is free to choose its own numbering format, as long as it guarantees uniqueness and continuity within each series. A common approach combines a prefix, a period, and a running counter, for example:

• FV/2026/07/001 -- prefix for "faktura VAT", year and month, sequential counter
• FV-2026-001 -- a yearly counter without a monthly break
• PROMO/2026/07/001 -- a separate series reserved for a specific document type

Several numbering series can run in parallel, for example:

• One series per document type (standard invoices, proforma invoices, receipts)
• One series per branch or point of sale
• A counter that resets every month or every calendar year

The only requirement is that each series, taken on its own, stays continuous and unambiguous -- mixing formats across different series is fine, as long as no series skips or repeats a number.

Invoice Number vs. KSeF Reference Number

The invoice number (numer faktury) printed on the document is chosen by the seller according to the numbering rules above, and it exists the moment the invoice is created -- before the document is sent anywhere.

The KSeF reference number (numer KSeF) is a different identifier. It is generated automatically by KSeF (Krajowy System e-Faktur, Poland's National e-Invoice System) only after the invoice has been transmitted and accepted by the system.

• The invoice number is chosen by the seller and must stay continuous within its series
• The KSeF reference number is assigned by KSeF itself and confirms the invoice is officially registered
• An invoice already has its own number when it is created; it receives a KSeF reference number only once KSeF has processed and accepted it

Biurko generates the next invoice number for each series automatically.

Corrective Invoices

Corrective invoices (faktury korygujące) typically use their own dedicated numbering series, kept separate from the series used for standard invoices. This keeps the audit trail clear: anyone reviewing the records can tell at a glance which documents are original invoices and which are corrections, while each series remains internally continuous.