An invoice number helps you, your customer and your accountant find exactly the same document. For an ordinary Belgian VAT invoice, Article 5 §1, 1° of Royal Decree No. 1 requires a sequential number within one or more series that uniquely identifies the invoice. The issue date must also appear.
The examples below turn that rule into practical checks. The number formats and workflow are organisational suggestions, not a legally prescribed template. For the other fields, see our checklist of mandatory invoice details.
Choose an identifiable structure
A simple structure could be 2026-0001, followed by 2026-0002 and 2026-0003. The year identifies the series; the final part increases. Write down your chosen structure so that a colleague or accountant can understand the same numbers.
| Example | Practical meaning |
|---|---|
2026-0041 → 2026-0042 |
Two successive numbers in the same annual series. |
A-2026-0041 and B-2026-0041 |
Two identifiable series with different complete numbers. |
Two different invoices both numbered 2026-0041 |
The same complete number no longer identifies just one invoice. |
Check the complete invoice number, not just its final digits. The legal provision allows multiple series, but additional series also make administration more complex. As a practical choice, only add them when you can clearly explain what each series is for.
A new year does not mean an automatic reset
The provision cited above does not prescribe an annual restart. If you choose a new annual series, check the transition deliberately. In our example, 2027-0001 distinguishes the new series from the earlier 2026-… series.
That differs from using 0001 again without any distinction when that complete number already identifies another invoice. Record when the new series starts and check the issue date. If unsure, work with your accountant to make the chosen approach fit your records.
Changing software: check before the first send
When switching, the visible format is only part of the check. As a practical precaution, establish which numbers are already in use and who can still issue invoices in the old application.
- Record the last number used in each existing series and the corresponding invoice date.
- Agree a changeover point and decide whether to continue a series or use a clearly distinguished one.
- Check the first number proposed by the new software before issuing an invoice.
- Compare the complete number with existing invoices, including those involving other applications or colleagues.
- After issuing the first real invoice, check that you can find it under the same number in your records.
Our e-invoicing changeover checklist also covers receiving invoices and access to earlier documents.
Found a duplicate or a gap?
First establish what happened: is it a draft, an invoice already issued, or a view of a different series? Do not change existing numbers simply to make a list look tidy. Preserve the context and discuss any correction with your accountant. This checklist does not determine which correction your particular case requires.
Before sending, check the number, series and date together. Looking for invoicing software for your Belgian business? Explore Cleero’s plans.