Your invoice numbering must be sequential, chronological and without gaps. That is what the Spanish Invoicing Regulations require: each invoice must have a unique number within its series, and there can be no skips or repetitions. If the Spanish Tax Agency (AEAT) reviews your invoices and detects gaps in the numbering, it may conclude that you have issued invoices you have not declared. The risk is not worth taking.
What the regulations say about numbering
Article 6 of the Invoicing Regulations (Royal Decree 1619/2012) states that every invoice must carry a sequential number within one or more series. The specific requirements are:
- Sequential numbering without interruptions within each series.
- The number may include letters, digits or a combination of both.
- You may use several series simultaneously, provided each one maintains its own sequential order.
- A number cannot be repeated within the same series in the same tax year.
What the regulations do not say is that you must start from 1 each year, or that you must use a specific format. You have considerable freedom in the design, but none whatsoever when it comes to sequential order.
Common numbering formats
There are two main approaches:
- Continuous numbering without year: you start at 1 and continue indefinitely. Invoice 1.547 may be from 2023 and invoice 1.548 from 2024. This is valid, but it makes it difficult to tell at a glance which tax year each invoice belongs to.
- Numbering with year: the most common format among self-employed (autónomo) workers is something like 2025-001 or F2025/001. When the year changes, you reset the counter. This is more convenient for keeping track and for reconciling with your quarterly returns.
If you use the year-based format and reset in January, make sure the software you use does this automatically. If you manage it manually or in a spreadsheet, note down the last number used in December so you do not make mistakes when you resume in January.
When to use separate series
A series is simply a prefix that groups a type of invoice. You are not required to have several, but in some cases it makes a great deal of sense:
- Series by document type: one series for ordinary invoices (A or F), another for simplified invoices or receipts (T or S), another for credit notes (R).
- Series by client or market: for example, one series for domestic clients and another for clients in the EU.
- Series by project or activity: if you have two very distinct lines of business, this can help you separate your invoicing.
The important thing is that each series has its own internal sequential order and that you do not mix documents of different natures within the same series without a clear rationale.
What to do at the end of the year
If you use year-based numbering, on 1 January you simply reset the counter for each series. Invoice F2024/087 is followed by F2025/001, and that is perfectly correct because they are different series (the year forms part of the identifier).
If you use continuous numbering, you do not need to do anything: you carry on from where you left off.
A common mistake: issuing an invoice in January with number 1 when you already have invoices from the previous year with that same number and the same series. If your series is simply “F” and you already have an F/001 from 2024, you cannot issue another F/001 in 2025. You must change the prefix or use the year as part of the number.
Credit notes and their numbering
Credit notes have their own series, separate from ordinary invoices. The reason is straightforward: they are documents of a different nature, and mixing them with regular invoices creates confusion in the accounts and in VAT (IVA) returns.
The standard practice is to use a series with the prefix R (R2025/001, R2025/002…). Each credit note must make explicit reference to the original invoice it corrects: number, date and reason for the correction.
There are two types of credit note:
- By substitution: you cancel the original invoice and issue a new one with the correct details.
- By difference: you issue a credit note that records only the difference (positive or negative) relative to the original.
In both cases, the credit note carries its own sequential number within the R series, and the numbering cannot have gaps, just as with any other series.
A practical example with numbers
Imagine you are a freelance designer. You decide to use the following series:
| Series | Document type | Example number |
|---|---|---|
| F2025 | Ordinary invoices | F2025/001, F2025/002… |
| T2025 | Simplified invoices (receipts) | T2025/001, T2025/002… |
| R2025 | Credit notes | R2025/001, R2025/002… |
In March you issue F2025/012. You realise you entered the wrong amount: it should have been 1.200 € but you put 1.000 €. You issue credit note R2025/001, state that it corrects F2025/012 due to an error in the amount, and record the difference of 200 € plus the corresponding IVA. The next ordinary invoice will be F2025/013, and the next credit note (if there is one) will be R2025/002.
In January 2026 you start from scratch: F2026/001, T2026/001, R2026/001. The 2025 series are closed.
Mistakes to avoid
- Leaving gaps in the numbering (due to invoices deleted or cancelled without issuing a credit note).
- Repeating numbers within the same series.
- Issuing invoices out of chronological order within the same series.
- Mixing ordinary invoices and credit notes in the same series.
- Changing the numbering format mid-year without a clear rationale.
If you need to cancel an invoice you have already issued, do not delete it: issue a credit note that cancels it. Deleting invoices creates gaps, and gaps attract the attention of Hacienda.
How Kullqi manages numbering for you
Kullqi automatically assigns the sequential number for each invoice within its series, without gaps and without you having to think about it. You can configure your own series (ordinary, simplified, credit notes) and the programme keeps the counter going on its own. Moreover, because each invoice is chained and submitted to the AEAT via Veri*Factu, you cannot delete or modify an invoice that has already been submitted without leaving a trace: if you need to correct it, the workflow itself guides you to issue the corresponding credit note. Sequential order is thus guaranteed by design.
Frequently asked questions
Can I reset the numbering to 1 each year?
Yes, provided the year forms part of the series identifier. If your series is “F2024” and the next one is “F2025”, they are different series and each can start at 001. If your series is simply “F” and does not include the year, you cannot reset the counter because you would generate duplicate numbers within the same series.
What happens if I accidentally skip a number?
An isolated gap does not automatically result in a penalty, but it can raise questions during an inspection. If the gap is due to an invoice you issued and then deleted, the correct approach is to keep the original invoice and issue a credit note to cancel it. If it was a system error, keep any evidence that explains it. In any case, consult your accountant if you are unsure how to regularise the situation.
Do proforma invoices or quotes share numbering with invoices?
No. Quotes and proforma invoices are not invoices and have no tax effect, so they carry their own independent numbering. Only invoices (ordinary, simplified and credit notes) are subject to the requirements of the Invoicing Regulations.
Can I have separate series for Spanish clients and EU clients?
Yes, and in many cases it is a good idea. Invoices to businesses in other EU countries have specific tax characteristics: B2B services are transactions not subject to Spanish IVA under the reverse charge mechanism, and intra-Community supplies of goods are exempt. Having a separate series (for example, EU2025) makes it easier to identify them quickly when preparing Form 303 or Form 349. Consult your accountant on the best way to organise this according to your activity.
If you want to stop worrying about numbering and focus on your work, try Kullqi free for 14 days with no card required at https://app.kullqi.com/.