What are the Invoice Numbering Rules?
Written by Arnon Shimoni
✓ Expert
Last updated on:
Invoice numbering is the rule that gives every invoice a unique, sequential identifier. Article 226(2) of the VAT Directive requires "a sequential number, based on one or more series, which uniquely identifies the invoice", and every tax authority in Europe reads that as: no duplicates, no gaps you can't explain, and no going back.
The reason is auditability. If your invoices run from 2026-00001 to 2026-04812, an inspector knows you issued 4,812 invoices and can ask to see any of them. A missing number means a missing invoice, which means unreported revenue until proven otherwise.
At a glance | Invoice numbering |
|---|---|
Legal basis | VAT Directive Article 226(2), plus national rules on gaps and series |
Must be | Unique, sequential, chronological within a series |
Series allowed | Yes, one or more. Per legal entity, per year, per document type, per country |
Gaps | Avoid. Where unavoidable, document why |
Credit notes | Own sequential number, usually a separate series |
Reset | Allowed at year boundary if the year is part of the number |
Assigned when | At finalisation, never at draft |
What the rules require
Unique. No two invoices from the same issuer can share a number. That includes across legal entities within a group if they share a system, which is why entity codes end up in the number.
Sequential. Each number follows the previous one in its series. 1001 is followed by 1002. Random or hash-based identifiers don't qualify, even though they're unique.
Based on one or more series. You don't need one global counter. You can run separate series per billing entity, per country, per year, per product line or per document type, as long as each series is itself sequential and the invoice number makes clear which series it belongs to.
Chronological. Within a series, a higher number should carry a later or equal issue date. An invoice dated 3 March can't sit between two invoices dated in April. This is where systems that assign numbers at draft time get into trouble.
National rules tighten this further. Germany's §14(4) UStG and the Netherlands' Article 35a Wet OB both require consecutive numbering. France requires a continuous, chronological sequence without breaks. Italy requires progressive numbering that identifies the invoice uniquely. Spain requires correlative numbering within each series. Portugal goes furthest: invoice numbers are generated by certified software and carry a hash chaining each invoice to the previous one.
Gaps
A gap is a number that was reserved but never corresponds to an issued invoice. They come from:
Assigning the number to a draft, then deleting the draft
Voiding an invoice after numbering
Two systems or two people drawing from the same counter
Reissuing a corrected invoice under a new number without a credit note for the old one
Gaps aren't automatically illegal, but each one is a question in an audit. The clean approach is to assign the number only at the moment the invoice becomes final, and to keep voided or cancelled documents on file under their number with a reason. A voided invoice that exists with a zero value and a note is fine. A number that corresponds to nothing is not.
Designing a numbering scheme
A good scheme reads like a sentence. INV-NL-2026-03-000412 tells you it's an invoice, from the Dutch entity, issued in March 2026, the 412th that month. Components you'll typically combine:
Component | Why include it |
|---|---|
Document type prefix (INV, CR) | Separates invoices from credit notes so each has its own series |
Legal entity or country code | Keeps each entity's sequence unbroken and separate. Required when entities file separate VAT returns |
Year (and optionally month) | Lets the counter reset each period without creating duplicates |
Zero-padded counter | Sorts correctly and signals how many you issue |
Prefix, year, month and a zero-padded counter: the number reads as a sentence.
Things to avoid: encoding the customer ID in the number (it leaks how many customers you have and breaks chronology when you backfill), starting at 1 (it tells your first customer they're your first customer), and changing the format mid-year without a documented cutover.
Numbering and the invoice lifecycle
The number should be the last thing an invoice gets. In a recurring billing system an invoice typically moves through:
Open. The billing period is running and charges are accumulating. No number.
Draft. The period has ended. Late usage events, adjustments and reviews land here. Still no number.
Final. The invoice is locked, numbered and sent. From this point it can't change. Corrections are credit notes.
Numbering at step 3 means the sequence stays chronological and gap-free however many drafts are edited or voided along the way. The invoice date should be tied to the same moment, or to a rule (last day of the period, first moment after it) that also keeps dates and numbers moving in the same direction.
Where Solvimon fits
Solvimon assigns the invoice number when an invoice goes FINAL, never to a draft, so the sequence is chronological and gap-free by construction. The default format is INV-YYYY-MM-00000x, with a configurable prefix and placeholders for the billing entity's reference, its registered country or custom fields, so a group with Dutch and Estonian entities gets INV-NL-2026-08-00001 and INV-EE-2026-08-00001 as two independent unbroken series. Credit notes run in their own CR series with separately configurable numbering, and you can choose a single sequence across the account or a sequence per customer. If your current setup has a spreadsheet somewhere tracking which number comes next, talk to one of our experts.
FAQ
Can I use more than one invoice number series?
Yes. The Directive says "one or more series". Each series must be sequential and identifiable from the number itself.
Can I reset the counter every year?
Yes, as long as the year is part of the number so that 2025-00001 and 2026-00001 are distinguishable.
Is a gap in the sequence illegal?
Not in itself under EU law, but tax authorities treat gaps as potential unreported sales. Keep cancelled documents on file under their number so every number is accounted for.
Do credit notes share the invoice sequence?
They can, but a separate series with a CR or CN prefix is clearer and is what most systems and auditors prefer.
Should the invoice number include the customer's ID?
No. It breaks chronology, leaks business information and adds nothing the invoice doesn't already carry.
What about a simplified invoice?
The Directive doesn't require a number on a simplified invoice, but most national rules do, and every system issues one anyway.
This page was checked against the VAT Directive and national tax authority guidance in October 2026. It is a general explanation and doesn't replace advice from your tax adviser.
Related
EU invoice requirements. The full list of mandatory invoice contents.
Credit note. Why issued invoices are never edited.
Multi-entity billing. One sequence per legal entity.
Invoice issuance deadline. The date rules that have to move with the numbers.
Billing engine. The system that owns the counter.
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