Invoice Numbering: Series, Gaps and BT-1

Invoice numbering looks like the least interesting requirement in the VAT Directive. It is one line of Article 226: a sequential number, based on one or more series, which uniquely identifies the invoice. Most businesses satisfied it decades ago and have not thought about it since.

Structured invoicing brings it back into view for a practical reason. When invoices are cleared or reported, the number is no longer only a reference printed on a page. It is a key in somebody else's database — the platform's, the buyer's, the administration's — and a duplicate is no longer an oddity for the auditor to query. It is a document a platform may refuse and that every downstream reconciliation will trip over.

What the rule permits

The Commission's explanatory notes on the invoicing rules are short on this point and generous. The number may be based on one or more series; it may include alphabetic as well as numeric characters; and the choice of series lies with the business. The notes give examples — a series per branch, per type of supply, per customer — and state expressly that separate series cover self-billed invoices and invoices issued by third parties on the business's behalf.

Two things are fixed, and they are the words doing the work. The number must be sequential within its series. And, taken with the series, it must uniquely identify the invoice. Everything else is design.

Where it lives in the document

In the European semantic model, the invoice number is BT-1, and it is mandatory. It sits alongside BT-2, the issue date, and between them they are what everyone else will use to identify the document. A credit note or corrective invoice refers back to the document it corrects through BG-3, Preceding invoice reference, which must contain BT-25, the earlier invoice's number. That link is what lets a buyer, a platform or an auditor pair the correction with the original, and it is described from the correction side in corrections and credit notes.

The format imposes no pattern on BT-1. A national profile may restrict its length or characters, and that is a constraint to check before a numbering design is fixed, not after.

The worked design

Consider a group with two legal entities in the same country, each issuing invoices from three sources: the ERP's order-to-cash module, a separate billing system for subscriptions, and a small volume of manual invoices raised by the finance team. One entity also receives self-billed invoices from two large customers.

The design follows from that count. Give each issuing system its own series, distinguished by a prefix, so that no two systems can ever issue the same number: ERP-2026-000001, SUB-2026-000001, MAN-2026-0001. Give each self-billing arrangement its own series too, agreed with the customer who operates it — that customer is the one allocating the numbers, and the business needs to be able to identify which arrangement a self-billed invoice belongs to. The mechanics of those arrangements are in self-billing under mandatory e-invoicing.

Whether credit notes take their own series is a choice. It makes them easier to identify at a glance; it adds a series to govern. Either is compliant.

Gaps, cancellations and rejections

A gap in a sequence is not in itself prohibited by the Directive's wording, but it is exactly what an auditor asks about, and the answer has to be available. Keep a record of numbers that were allocated and not used — a draft abandoned, a document voided before issue — with the reason. That record is part of the audit trail, and it is short work if kept as it happens and long work if reconstructed later.

Clearance adds a harder case. When a platform rejects a document, has it been issued? If not, is its number consumed? The answer is national, and it varies with how a country defines the moment of issue. The rule for each country you invoice in should be established from its own specification and written into the numbering design, rather than left for the first rejection to decide.

A short checklist