Key takeaways
- Every invoice must carry a sequential number, based on one or more series, that uniquely identifies it.
- The rule comes from Belgian VAT rules, which transpose Article 226 of Directive 2006/112/EC.
- A continuous numbering, with no gaps, is what proves that no invoice was removed from the sales register.
- E-invoicing does not change the rule: the number remains a mandatory field of the Peppol invoice.
Invoice numbering, a legal obligation, not an internal convention
Invoice numbering is not an internal convention that each business organises as it likes: it is a mandatory detail of the invoice, on the same footing as the date or the VAT number. In Belgium, an invoice that does not clearly identify each document by a unique number is not a proper invoice, and a missing detail can weaken your customer's right to deduct the VAT.
The rule has a precise source. Belgian VAT rules, which govern the mandatory invoice details, transpose Article 226 of Directive 2006/112/EC. That text requires "a sequential number, based on one or more series, which uniquely identifies the invoice". Every word counts, and this article explains which.
It sets out what the law requires, how to build a numbering series that holds over time, how to treat credit notes, and what the arrival of mandatory e-invoicing changes here.
What the law requires: a unique, sequential number, per series
Three requirements sit inside the legal wording. The number must first be unique: two invoices can never carry the same number. It must then be sequential, meaning it follows an order, without the series jumping from one document to the next for no reason. And it may rest on one or more series, which leaves real freedom of organisation.
That number is the one under which the invoice is recorded in the supplier's sales register, the record where issued invoices are logged. It is this link between the number and the register that gives continuity its meaning: an unbroken series lets anyone check that no invoice went missing between two numbers.
The criteria of compliant numbering
Uniqueness
No number is ever reused, including from one year to the next within the same series.
Sequence
Numbers follow a defined order; continuity is easy to demonstrate.
One or more series
You may run several series in parallel, each continuous on its own.
Assigned at issue
The number is set when the invoice is drawn up, not retroactively.
Building a numbering series that holds over time
The law does not impose a number format: it imposes properties. You are therefore free to choose the form, as long as uniqueness and sequence are respected. Two approaches dominate in practice, and the choice mostly depends on your volume and your filing habits.
The first is the continuous series: invoices follow on indefinitely, never restarting from 1. The second is the annual series: the numbering embeds the year and restarts each 1 January, for example 2026-001. This second form is allowed precisely because the law speaks of "one or more series": each year is a continuous series in itself.
| Continuous series | Annual series | |
|---|---|---|
| Restarts at 1 each year | ||
| The year is readable in the number | ||
| Uniqueness guaranteed over the life of the business | ||
| Allowed by 'one or more series' |
One rule outweighs the choice of format: do not mix logics mid-course. Switching from a continuous series to an annual one mid-year creates needless ambiguity. Set your scheme, document it, and let your software apply it without manual intervention. In practice, it is the manual keying of a number that produces duplicates and gaps.
Gaps in the numbering and how to avoid them
A missing number in a series does not cancel the invoices around it, but it raises a question you must be able to answer: where did invoice no. 47 go? During an audit, the VAT authority reads the continuity of the series as a guarantee that no invoice was removed. An unexplained gap shifts the burden onto you.
Gaps almost always have the same origin: a numbered draft then deleted, a cancelled invoice whose number is not reused, a double entry corrected by hand. The remedy is structural rather than disciplinary.
Credit notes and separate series
A credit note corrects or cancels an already issued invoice. It, too, carries a unique sequential number. You have a choice: record it in the same series as your invoices, or reserve a series of its own for it, for example prefixed CN. Both approaches are valid as long as each document stays uniquely identified.
A separate series for credit notes has a practical benefit: it visually separates the documents that increase your turnover from those that reduce it, which eases the accounting reconciliation. The link between the credit note and the corrected invoice, though, does not run through the series number but through an explicit reference to the original invoice. For the capping and issuing rules, see the electronic credit note.
Continuous numbering, with no gaps and no effort
YouInv automatically assigns a unique sequential number to every invoice and credit note, and manages your series for you.
What e-invoicing changes
The mandatory B2B e-invoicing obligation in Belgium does not change the numbering rule: it makes it even harder to avoid. On the Peppol network, the invoice number is a structured field of the document, carried by a dedicated element of the EN 16931 model (the invoice identifier). Without it, the document would not be a valid invoice, and a receiving system would reject it.
The difference is that the number is no longer a string you type at the top of a PDF, but a value your software records and transmits automatically. You keep control over the format and the series; you lose the keying errors. That is where moving to a compliant tool really changes things, well beyond numbering alone: the correct data is produced once, then reused. For the general framework, read the legal basis of B2B e-invoicing.
Further reading
- Mandatory invoice details in Belgium: the full list the number is part of.
- Invoicing rules in Belgium: the framework of obligations around the invoice.
- The UBL 2.1 format explained: where the number sits in the e-invoice.
The reference source prevails: Article 226 of Directive 2006/112/EC and the VAT rules published by the FPS Finance.
Is invoice numbering mandatory in Belgium?
Yes. Every invoice must carry a sequential number, based on one or more series, that uniquely identifies it. This detail is one of the mandatory invoice details set by Belgian VAT rules, which themselves transpose Article 226 of Directive 2006/112/EC.
Can you restart the numbering each year?
Yes. The law allows numbering 'based on one or more series'. An annual series that restarts at 1 every 1 January (for example 2026-001) is therefore allowed, provided each number stays unique and the series is continuous within the year.
Is a gap in the invoice numbering a problem?
A continuous series, with no missing number, is what shows that no invoice has been removed. A gap does not cancel an invoice, but it draws attention during a VAT audit and you must be able to explain it. Software that assigns the number automatically avoids the issue.
Do credit notes follow the same numbering as invoices?
A credit note also carries a unique sequential number. It can belong to the same series as your invoices or to a separate series of its own; what matters is that each document is uniquely identified and recorded in the relevant register.
Who assigns the invoice number, the seller or the customer?
The issuer of the invoice assigns the number, from their own series. Under self-billing, where the customer draws up the invoice on the supplier's behalf, the number follows the series agreed between the parties, but stays unique.
