Invoice Series and Numbering: Rules, Mistakes and Best Practice Under the SIF

Invoice Series and Numbering: Rules, Mistakes and Best Practice Under the SIF

Invoice numbering used to feel like a footnote — until Spain's anti-fraud invoicing regulation came fully into force (companies since 1 January 2026, self-employed workers since 1 July 2026). Every invoice now generates a billing record with an SHA-256 chained hash, and that chain turns any numbering shortcut into visible evidence. Time to get the rules straight.

The base rule: strictly sequential within each series

Spanish invoicing rules require numbering to be sequential and gap-free within each series: 1, 2, 3… no skips, no repeats. You may run several series, but inside each one the sequence must be unbroken. The identifier format is up to you (A-2026-0001, INV26/001), the order is not.

This is stricter than what many international businesses are used to. In several countries an invoice number merely has to be unique; in Spain it must also be consecutive per series. With a compliant SIF this stops being a manual discipline: the system assigns the number automatically and chains each record to the hash of the previous one. There is no way to slip in a back-dated invoice without the chain showing it.

When to open separate series

Some series are mandatory, others optional:

• Mandatory: a dedicated series for corrective invoices (credit notes). Corrections never live inside the ordinary series.
• Mandatory: separate series when invoices are issued by the customer (self-billing) or by a third party on your behalf — cooperatives, platforms, accounting firms.
• Optional but sensible: series per branch, sales channel (shop vs online), transaction type or business line. They make internal control and audits easier.

Practical advice: fewer series is better. Every series is one more sequence to keep clean; only open the ones that answer a real need.

What if a gap or a skipped number appears?

With spreadsheets or legacy software, the classic reflex was to "fill" the gap by manually creating an invoice with the missing number. Under the SIF that is neither possible nor necessary:

• The SIF blocks out-of-sequence insertions: each record's hash embeds the previous one, so a squeezed-in invoice would visibly break the chain.
• If the gap comes from a cancelled or erroneous invoice, the correct path is a cancellation record or a corrective invoice in its own series — never renumbering.
• If a genuine skip occurs (an operational glitch, for instance), document it. The SIF's event log already records what happened and when, and that traceability is your best defence in a tax review.

Keep the stakes in mind: using invoicing software that allows this kind of manipulation carries penalties of up to 50,000 euros per tax year (article 201 bis of the Spanish General Tax Law).

Year-end: reset or continue?

Both are valid. You can restart numbering every year (the common pattern: 2026-0001, then 2027-0001) or keep one continuous sequence. If you restart, include the year in the series so no two invoices ever share the same full identifier. What matters is deciding the rule before 1 January and applying it consistently — the SIF handles the rest.

Switching software mid-year without breaking the sequence

Changing invoicing systems in March or September is perfectly legal. Two clean approaches:

1. Continue the series: configure the new system to start at the next number (last invoice A-2026-0341, first new one A-2026-0342). The new SIF's hash chain starts there; keep the old system's records for the statute-of-limitations period. 2. Open a fresh series from the cut-over date (say B-2026-0001) and close the old one. This is the simplest option and the one that raises fewest questions in an inspection.

What you must never do: re-issue in the new system invoices already issued in the old one, or let both systems keep emitting the same series in parallel.

Classic spreadsheet mistakes the SIF eliminates

• Two invoices with the same number because two people edited the file.
• Invoices dated last week but created today.
• Gaps back-filled later with recycled or invented transactions.
• Series improvised mid-year with no documented rule.
• Deleting an invoice and reusing its number.

Each of these habits was invisible in a spreadsheet. With chained hashes, QR codes and an event log, they are exactly what an inspection spots first. The upside: let the SIF manage series and numbering, and this whole slice of compliance runs itself.