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KSeF Invoice Reference Number – Key Information
- Paweł Gorzelec
- Nov 6
- 3 min read
As the mandatory implementation of the National e-Invoice System (KSeF) approaches in 2026, many entrepreneurs are wondering how to understand the identification number assigned to invoices by the system. What exactly is the KSeF number, what is its structure, and when is it assigned to an invoice?
What is a KSeF Invoice Number?
The KSeF number is a unique 35-character identifier assigned to every invoice issued within Poland’s National e-Invoice System. It is automatically generated by the Ministry of Finance’s system and enables each document to be uniquely identified in circulation. Importantly, this number does not replace the traditional invoice number assigned by the seller — both numbers exist simultaneously and serve different purposes.
Structure of the KSeF Number
The KSeF number consists of four logical parts:
first 10 characters – the seller’s Tax Identification Number (NIP),
next 8 characters – the invoice issue date in the format YYYYMMDD,
next 12 characters – two technical sections (6 characters each) generated automatically,
last 2 characters – a control sum, also generated automatically.
This structure ensures that every document can be precisely identified in the system — by both taxpayers and tax authorities.
When Is the KSeF Number Assigned?
The number is assigned the moment the invoice is accepted by the KSeF system, which also confirms its successful issuance. For entrepreneurs, this means that:
the invoice has been transmitted and accepted by the system,
the date of KSeF number assignment is considered the date of receipt by the buyer (if the buyer has a NIP).
An exception applies to invoices issued offline or to consumers — in those cases, the date of actual delivery is regarded as the moment of receipt.
Invoice Number vs KSeF Number – What’s the Difference?
In practice, businesses may encounter three different types of invoice identifiers:
KSeF number – system-generated identifier used for tracking the document in the national database,
Invoice number – assigned by the seller in accordance with the company’s internal numbering rules (e.g. INV/11/2025),
Accounting number – internal reference used in the company’s books or accounting records.
Note: The KSeF number does not have to appear on the invoice itself — it is visible directly in the system.
Invoice Issuance Modes and Number Assignment
The National e-Invoice System allows several modes of invoice issuance, and the process of assigning a KSeF number depends on the mode used:
Online mode – the invoice is issued directly in the system and immediately receives its KSeF number (default and recommended mode).
Offline24 mode – the invoice is issued outside the system (e.g., due to lack of Internet access) and must be uploaded to KSeF no later than the next day.
System downtime mode – when the system is unavailable due to a technical issue on the Ministry’s side (e.g., maintenance). In such cases, invoices can be issued outside the system but must be submitted to KSeF within 7 days from the end of the downtime, as announced in the Ministry’s official notice.
Can a KSeF-Numbered Invoice Be Edited?
No. Once a KSeF number has been assigned, the invoice becomes a registered document and its content cannot be altered.Any necessary corrections can only be made through a corrective invoice.
Summary
The KSeF number is a crucial element of the electronic invoicing process in Poland — it confirms that an invoice has been properly issued and accepted by the system. While it does not replace the traditional invoice number, it serves as an essential verification and control tool within the new e-invoicing model.
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