Rules on series, sequential numbering and format under RD 1619/2012.
Invoice numbering in Spain is not something you can organise as you please. The Invoicing Regulation (RD 1619/2012) sets clear rules on how invoices must be numbered, when you need to use separate series and what sequential numbering means. With the introduction of Verifactu, these rules become even more important, because the Tax Agency will be able to automatically detect any irregularity in the sequence.
Article 6.1.a of RD 1619/2012 establishes that numbering within each series must be sequential. This means that invoices must follow a sequential order with no gaps — 1, 2, 3, 4 — without numbers being deleted or inserted after the fact.
The format, however, is free. The law does not impose a specific number of digits or a particular prefix. You can number as 001, as 2025/001, as A-001 or however you prefer, as long as the sequential order is clear. Facturas simplificadas follow exactly the same rules within their own series.
A point that causes much confusion is whether numbering must be reset every 1 January. The answer is that it is not mandatory — the law only requires sequential order within each series. Resetting each year is a very widespread convention, but not a legal obligation.
Although series are optional in most cases, there are three situations in which the law requires using a specific series separate from ordinary invoices.
The first and best known is for facturas rectificativas. Article 6.1.a establishes that they must always be issued in their own series. An ordinary invoice and a factura rectificativa cannot share the same series prefix — they must be different series.
The second is when the invoice is issued by the recipient on behalf of the supplier (known as invoicing by the recipient). And the third is invoices for intra-Community supplies subject to special taxes. In both cases, the reason is the same — the Tax Agency needs to be able to distinguish these documents from the ordinary invoicing flow.
In addition to the mandatory series, you can create additional series when justified. Article 6.1.a of RD 1619/2012 expressly allows this when the issuer has multiple establishments, when they carry out operations of different natures, or when they issue different types of invoices.
In practice, the most common uses are: one series per establishment (one for each office or premises), one series per type of operation (services, products, training), or a series that includes the fiscal year in its format (2025-001, 2025-002). Any of these options is valid as long as each series maintains its own sequential numbering.
With the entry into force of Verifactu, numbering takes on an importance that was previously more theoretical than practical. Every invoicing record that the software sends to the AEAT includes the invoice series and number as identifying data. But more importantly, records are chained together using cryptographic hashes — the digital fingerprint of each new record includes the previous one.
This means that any gap, alteration or deletion in the invoice sequence will be automatically detectable by the Tax Agency, without the need for an on-site inspection. If previously a gap in the numbering could go unnoticed for years, with Verifactu the anomaly is recorded the moment it occurs.
CokuApp manages your invoice numbering automatically, with configurable series and guaranteed sequential numbering.