Invoice Numbering: Rules, Structure and Most Common Mistakes
Sequential, unique and without gaps — the rules for invoice numbering sound simple, but in practice things often go wrong.
Invoice numbers may seem like a trivial administrative formality, but the Dutch Tax Authority (Belastingdienst) sees it differently. A missing or duplicate invoice number can lead to questions during an audit and — in the worst case — a correction to your VAT return. Here are the rules and the smart choices.
Legal Requirements (VAT Act Article 35a)
- Sequential. No gaps in the sequence. 2026-001, 2026-002, 2026-003 — not 2026-001 then 2026-005.
- Unique. Each number may only appear once, even across years (in combination with the year).
- Logically structured. An auditor should be able to quickly see how many invoices you've issued.
The legal minimum requirement: a unique sequential number. The format is in principle free, but the Tax Authority prefers a recognizable system.
Recommended Formats
The three most common styles, with their pros and cons:
- 2026-001. Year + sequential number. Advantage: resets annually; you quickly see how many invoices you've issued in a year. Disadvantage: in January you have '2026-001' without context of when.
- F2026010001. Letter + year + month + sequential number. Advantage: super structured, alphabetically sortable. Disadvantage: long and ugly on the invoice.
- 0001. Pure sequential numbering from your start. Advantage: simple. Disadvantage: after 5 years you have number 4823 — not ideal for audits.
When Can You Start Over?
Many entrepreneurs restart at 001 each year (within a year-tag prefix). That's allowed, as long as the format makes clear which year it is — so 2026-001 instead of just "001".
However, you must never skip a number, even if you make a mistake. Did you send an incorrect invoice? Issue a credit note with the next number in the sequence; don't discard the first one.
Multiple Sequences: Is That Allowed?
Yes. Some entrepreneurs use separate sequences for:
- Sales invoices (F2026-001, F2026-002...)
- Credit notes (C2026-001, C2026-002...)
- Different business units (NL-2026-001, BE-2026-001...)
This is allowed, as long as each sequence is internally sequential. The Tax Authority looks at the logic per sequence.
Common Mistakes
- Discarding an invoice because the customer rejected it. Wrong: the invoice has been sent, the number has been issued. Issue a credit note for the full amount, then invoice again with a new number.
- Two monthly subscriptions with identical numbers. A recurring invoice must also get a unique number — not "2026-May" for two years in a row.
- Excel error in the manually maintained list. Jumping numbers, skipped ones. Reason to stop doing this in Excel.
- Test invoices in production. If you create a test invoice in your live accounting, you'll need to issue a credit note for it later or correct your numbering.
How Invoisio Solves This
We generate invoice numbers automatically in the sequence you choose (year tag + counter, with or without F-prefix, with optional month). You can change the format, but existing numbers remain unchanged. For each invoice, we verify that the number is unique — errors are technically impossible.
For multi-company setups (a holding with subsidiaries), we support separate sequences per business entity, all automatically correct and sequential.
Was this helpful?