The Audit That Started With Invoice Number 47
A Berlin-based freelance designer went through a Finanzamt audit in 2022. The trigger was routine. The damage was not: non-sequential invoice numbers — a gap between invoice 46 and invoice 48, caused by a deleted draft — gave the tax office grounds to question the completeness of her records. Input VAT claims across 18 months were suspended pending full documentation. The resolution took four months and cost more in accountant fees than the disputed VAT.
A sequential numbering system costs nothing to implement. Getting it wrong is expensive.
Rule 1: Unique, Sequential, No Gaps
The EU VAT Directive requires that invoices be numbered in a continuous sequential series. The exact wording (Article 226, point 2) requires a "sequential number, based on one or more series, which uniquely identifies the invoice."
What this means in practice:
- Every invoice number must appear exactly once across your entire business history
- Numbers must ascend without gaps — 001, 002, 003, not 001, 003, 004
- If you void an invoice, the number is retired. Do not reuse it. Do not skip and pretend it never existed — record it as voided.
Rule 2: Restarting Each Year Is Optional, Not Required
Many freelancers and small businesses reset their invoice counter at the start of each calendar year. This is permitted and often makes sense for bookkeeping. However, it is not legally required.
If you do restart annually, use a year prefix to prevent ambiguity. Invoice 001 from 2025 and Invoice 001 from 2026 are only distinguishable if the year is embedded in the number.
Popular Invoice Number Formats
| Format | Example | Pros | Cons |
|---|---|---|---|
| Simple sequential | 0001 | Clean, minimal | No year context; confusing over multiple years |
| Year-prefixed | 2026-001 | Clear year context; easy to audit | Must reset or maintain very high numbers |
| Year-month prefixed | 2026-02-001 | Precise; useful for high volume | More complex; overkill for low-volume freelancers |
| Client-prefixed | ACME-2026-001 | Useful for client reference | Risky if sequences overlap; must stay globally unique |
| Custom prefix | INV-2026-001 | Professional appearance | No functional advantage over year-prefixed |
Recommendation for most freelancers: Year-prefixed format (2026-001). It is clean, auditable, widely understood, and simple to maintain manually or with a tool.
Rule 3: Credit Notes Must Reference the Original
When you issue a credit note — to cancel, correct, or partially reduce an invoice — the credit note must explicitly reference the original invoice number. Without that reference, the credit note is not traceable and may not be accepted by the client's tax authority for input VAT purposes.
Example credit note reference: "This credit note cancels Invoice 2026-014 in full."
Issue the credit note with its own sequential number in your standard series. Credit notes are part of the same sequence as invoices.
Rule 4: Keep a Register
You are not legally required to keep a separate invoice register, but you must be able to demonstrate that your sequence is complete and continuous. In practice, this means maintaining a record — a spreadsheet, an accounting system export, or a tool log — that shows every invoice number issued, the date, the client, and the amount.
A basic spreadsheet with five columns (Number, Date, Client, Amount, Status) is sufficient. The goal is that if a tax inspector asks for Invoice 2026-031, you can produce it and show it sits correctly between 2026-030 and 2026-032.
What Not to Do
- Starting at 1000 to look established. Tax authorities are aware of this tactic. An audit that begins with Invoice 1000 from a brand-new business invites questions about invoices 1 through 999.
- Skipping numbers. Even accidentally. If you draft and delete invoice 2026-015 without issuing it, record it as voided in your register.
- Running separate sequences per client. This creates overlapping numbers and fails the uniqueness requirement.
- Reusing a number from a voided invoice. The number is retired the moment you assign it, regardless of whether the invoice was sent.
Keep Your Numbering Consistent With a Tool
Manual numbering fails when you are juggling multiple projects. The invoice generator on this site assigns sequential numbers automatically within a single unified series, tracks issued invoices, and flags if a number has been used before. It removes the administration risk and keeps your records audit-ready.