Be able to change or determine the invoice number yourself
complete
Sara Meert
complete
In Belgium, it is mandatory to number your invoices in a continuous, chronological and continuous order. This means that:
- Each invoice must have a unique number.
- The order of the invoice numbers must be logical and chronological (no jumps or gaps).
- You are not allowed to simply insert or delete invoices without this being visible in the numbering.
What does this mean in concrete terms?
- In principle, you can decide what the invoice number looks like (e.g. with year, customer code, etc.), as long as the numbering remains sequential and continuous.
- A cash register or accounting software (such as Optios) that does the numbering automatically helps to comply with this rule and avoid errors.
- The use of multiple series is allowed, for example one series for online sales and one for retail sales (e.g. 2025-W001, 2025-O001), as well as a series from you working with optios (OPT2025-001) as long as each series also remains internally chronological and logical and the system can be explained to the tax authorities.
Example of valid numbering: 2025-001, 2025-002, 2025-003,...
OR: F2025-001-W, F2025-002-W (store), and F2025-001-O, F2025-002-O (online)
Summary:
✅ You can choose a numbering yourself
✅ You must keep the numbering logical, continuous and chronological
✅ Automatic numbering via software is not mandatory, but recommended
❌ You shouldn't just use random or manual numbering that causes gaps, duplicates, or ambiguity
If you are subject to VAT, correct invoice numbering is extra important for accounting and VAT returns.
However, you can always set the prefix in Optios, so it's also clear when you start billing with Optios.