Belgian VAT law defines a number of rules that an invoice must comply with.

In our "Everything You Need to Know About Invoices" series, in Part 1 we explained exactly what an invoice is and the legal requirements it must meet. Next, in Part 2, we discussed the history and evolution of the invoice, including the importance of electronic invoicing. In this third blog of the series, we take a closer look at the mandatory rules for invoice numbering and share some practical tips for handling it correctly and efficiently.

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Invoice numbering rules

An important rule is that each invoice must be assigned a unique number within the taxpayer's own records. In addition, the numbers must be assigned chronologically and without interruption.

• Unique: Each invoice number may occur only once, making invoice retrieval easy and error-free. Thanks to these unique numbers, Billtobox can effortlessly detect duplicate invoices.
• Chronological: Invoices are numbered according to the date of issue. A later invoice always has a higher number than an earlier one in the same period.
• Continuous: Numbering must be continuous, with no gaps or duplicates. Erroneous invoices remain in the series and are corrected with a credit note.

An additional rule is that an invoice numbering sequence never starts with 0, so the first invoice will always be numbered 1.

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5 tips for effiencient invoice numbering

1. Choose a clear structure

You can determine the composition of your invoice numbering, for example, a combination of letters, numbers and characters. Note that spaces are not allowed. Use separate ranges for different invoice types, such as:

• Invoices: FACT-001
• Debit Notes: DN-001
• Credit notes: CN-001
• Advance Invoices: VFACT-001

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2. Start over every fiscal year

You may start each fiscal year with new numbering, but this is not required. Adding the fiscal year to the numbering (e.g., FACT2024-073) can help identify invoices quickly and keeps the numbering clear. That way, everyone knows that FACT2024-073 is the 73rd invoice of fiscal year 2024.

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3. Match the length of your numbering to your needs

Estimate in advance how many invoices you expect to create so that the sequence number has enough characters. For example, if you expect more than 100 invoices, choose three-digit numbering (e.g. 001, 002, 101). This keeps the sequence logical and consistent.

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4. Keep it simple

Although you can incorporate your customer numbers or other information into your invoice numbers, it is better to keep the numbering organized. Too much detail makes it unnecessarily complicated and can cause confusion.

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5. Use different numbering ranges for separate activities

Do you work in multiple industries or have several activities? If so, use specific numbering ranges for each activity. For example, you can:

• F-A-006 for wine info evenings
• F-W-079 for the sale of wine

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This approach makes it easier for an accountant to keep separate journals.

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Invoice numbering with Billtobox

With Billtobox , you have the ability to enable automatic numbering or assign different number ranges to your invoices. Useful right?

You can read more about this in the following Helpdesk article: How do I set up the numbering of my sales invoices?