Invoice numbering looks like a trivial detail, but a number series set up properly saves you trouble in a tax audit, makes your own records easier to navigate and keeps you on the right side of the law. This article covers what the law requires, the formats you will come across, and how numbering works in Taxorio.
What the law requires
Section 29(1)(e) of the Czech VAT Act requires a tax document to carry a "registration number of the tax document". That number identifies the document unambiguously and has to meet the following:
- It must be unique — no two invoices may share a number.
- It must allow the document to be identified unambiguously.
- The law itself requires only an unambiguous registration number; a sequential series — continuous, without gaps, within one or more number series — is a recommendation that makes your records easy to prove at an inspection, especially for VAT payers.
For non-payers the requirements are looser, since the VAT Act does not apply to them directly. Sequential numbering is still worth keeping, because:
- it makes your documents easier to work through
- it reduces the scope for errors in your records
- if you register for VAT later, you do not have to change anything
What "sequential numbering" actually means
It does not mean your invoices have to run 1, 2, 3 without exception. It means there should be no unexplained gaps within your number series. If an invoice falls through, it is better to cancel it — by issuing a corrective document — than to delete it and leave a hole in the sequence.
For VAT payers this matters more. The tax office may check that your number series is complete, and gaps can raise the suspicion that some documents were never declared. That is why numbering without gaps — not expressly required by Section 29, but the safest way to prove your records are complete — is particularly important once you are registered.
The formats you will meet
Several formats are established in practice. None of them is prescribed by law — what matters is uniqueness and continuity.
YYYY-NNN
The most widespread format joins the year and a serial number with a hyphen: 2026-001, 2026-002, 2026-003…
Advantages:
- the year of issue is visible at a glance
- you restart at 001 every year, so the numbers stay short
- it sorts chronologically without effort
YYYYNNNN
The year and the number with no separator: 20260001, 20260002…
Advantages:
- purely numeric, which suits systems that do not accept special characters
- it works as a variable symbol for bank payments with no adjustment
A prefixed format
A prefix marking the type of document: FV-2026-001 for an issued invoice, FP-2026-001 for a received one, ZF-2026-001 for an advance invoice.
Advantages:
- the type of document is obvious immediately
- it suits a business running several number series
- accountants find it easy to follow
Why the format matters
A tax audit
During an audit the inspector goes through your documents and checks that they are complete. A clear format with the year and an unbroken sequence makes that quick and cuts down the questions. Chaotic or unsystematic numbering can drag the audit out.
Electronic filing through EPO
When you file the control report (DPHKH1) or the VAT return (DPHDP3), you state the registration numbers of the documents. A consistent format makes that easier and reduces the risk of a mistake. On the PRO plan Taxorio generates these exports automatically, with invoice numbers in its fixed YYYYXXXX format.
Talking to clients and accountants
When a client calls about "invoice 2026-042", you know at once which invoice they mean. A system with no year in the number — a bare "42" — would leave you working out which year it came from.
Numbering in Taxorio
Taxorio uses automatic numbering in a fixed format that you do not need to configure. Left to run on its own, the system maintains an unbroken series for you.
The fixed YYYYXXXX format
Ordinary invoices use the fixed YYYYXXXX format: a four-digit year and a four-digit serial number, with no separator.
- YYYY — the current year, for example 2026
- XXXX — the serial number padded with zeros to four digits: 0001, 0002 and so on
The first invoice of 2026 is therefore 20260001, the second 20260002, and so on. The format is purely numeric, so it works as a variable symbol for bank payments without adjustment.
Separate series for special documents
Advance invoices, credit notes and tax documents for an advance received have their own fixed prefixes and their own counters:
- ZAL20260001 — an advance invoice
- DOB20260001 — a credit note, that is a corrective tax document
- DDZ20260001 — a tax document for an advance received
The serial number
The serial number is assigned automatically within the year from a counter you can adjust in the settings; on an individual document you can also type your own invoice number. Left alone, the counter guarantees a continuous series with no duplicates and no gaps.
The variable symbol
Taxorio derives the invoice's variable symbol from the invoice number, so the two are consistent. That simplifies matching payments in the bank: the client enters the variable symbol and you find the matching invoice straight away.
The reset at the start of the year
The serial number resets automatically at the start of each calendar year. Because the year is part of the number, uniqueness survives the reset — 20250001 and 20260001 are two entirely different documents.
The reset happens on its own: the year in the number always matches the current year, so numbering carries over at the turn of the year and there is nothing for you to do.
Moving from another system
If you are coming to Taxorio from another invoicing tool, be aware that Taxorio uses its own fixed YYYYXXXX format and restarts the serial number at 0001 each year. The format cannot be changed, but the counter can be set so that the series continues from your previous system.
In practice that does not matter: the law does not require your number series to continue on from your previous software. It only has to be unambiguous within your own records; a continuous series is recommended. The natural moment to move is therefore the turn of the year, when you start a fresh series at 20260001 anyway. Documents from the old system remain valid in their original series.
The mistakes to avoid
- Duplicate numbers — two invoices sharing a number. Taxorio prevents this by incrementing the number itself.
- Erratic jumps — 001, 003, 007, 015 with no logic behind them. For VAT payers this invites questions in an audit.
- Overwriting numbers by hand — editing an automatically assigned number raises the risk of errors and duplicates.
- No year in the number — plain 001, 002 and so on make records harder to navigate and can produce duplicates across years.
- An over-elaborate format — something like "FV-POB-2026-A-001" is needlessly complex for a sole trader. A plain "YYYYXXXX" does the job.
What we recommend
- Use the YYYYXXXX format. It is clear, it carries the year and it is simple enough — and Taxorio uses it as a fixed format anyway.
- Let the numbering run automatically. That removes the risk of duplicates and gaps.
- Do not delete invoices. Cancel them instead, so the number series stays complete.
- Plan a migration for the turn of the year. A new series starting at 20260001 then lines up naturally with the calendar, and your old documents keep their original numbers. If you import old invoices, note that the import does not move the counter — set it manually after the migration.
- Leave the variable symbol alone. Taxorio derives it from the invoice number, which is what makes payment matching work.
Getting numbering right is a one-off job that repays itself in saved time and in a calmer tax audit. Set yourself up in Taxorio and you can stop thinking about it. If you need to void an invoice that has already gone out, see how to cancel an invoice with a credit note.
Taxorio scope: Taxorio provides invoicing and income/expense records for Czech sole traders. It is not full accounting or personalised tax advice. For an unsupported or unusual case, verify the treatment with a Czech accountant or tax adviser before filing.