Definition: The invoice number is a unique identifier for each invoice, used for identification and traceability. According to § 14 UStG, an invoice must, among other things, contain “a consecutive number with one or more numeric sequences that is assigned uniquely for the identification of the invoice.” For freelancers and small businesses the invoice number is therefore a central element of accounting and VAT documentation.
Legal basis
The most important legal bases for the invoice number come from the Value Added Tax Act (UStG), the Fiscal Code (Abgabenordnung, AO) and the GoBD (Principles for properly keeping and storing books, records and documents in electronic form and for data access).
- UStG (§ 14): Obligates the inclusion of a consecutive, unique number on invoices so that each invoice can be identified unambiguously.
- AO (§ 147): Regulates retention periods — invoices generally must be retained for ten years.
- GoBD: Set requirements for traceability, auditability and immutability of invoice data, including numbering ranges.
Requirements for the invoice number
In practice the requirements can be reduced to three core points: uniqueness, traceability (sequential), and documentation of the system. These aspects have a direct impact on daily bookkeeping.
Uniqueness
Each invoice number may only be assigned once. Duplicate numbers lead to queries from the tax office and complicate reconciliation in accounting. For multiple GmbH branches or business units it is advisable to use prefixes for clear allocation (e.g. BER-2025-0001 for Berlin).
Sequential numbering
The law does not require a gapless sequence without interruption, but it does require a traceable and sequential assignment. Gaps are permissible if they are documented (e.g. cancellation or manual invoice issued outside the system), but should be avoided to simplify audits.
Format and structure
The format may be chosen freely, but should be stable and traceable. Common elements are a year identifier, a prefix for business areas and a running number. Examples: 2025-000123, FREEL-25-045 or GMBH-MAIN-001. It is important that the format is documented and not changed arbitrarily.
Practical numbering systems and examples
In practice several simple systems have proven successful that meet the requirements and make daily work easier.
- Simple sequential: 0001, 0002, 0003… – good for sole proprietors without differentiation.
- Year-based: 2025-0001, 2025-0002… – separates yearly sequences and simplifies year-end closing.
- Area-based series: WEB-2025-001 for web projects, CONS-2025-001 for consulting – useful for multiple business areas.
- Client/branch prefix: BER-2025-001, MUC-2025-001 – for multiple locations.
Example table:
| Invoice | Number | Note |
|---|---|---|
| Consulting service January | 2025-001 | Year-based sequence |
| Website order | WEB-2025-002 | Area-based series |
| Credit note (cancellation) | 2025-001-CR | Reference to original number |
Handling gaps, credit notes and technical systems
Errors happen. What matters is how you document gaps and cancellations.
- Cancellations and credit notes: Reference the original invoice number (e.g. credit note for invoice 2025-010). Use separate series for credit notes or mark them clearly.
- Gaps in the numbering sequence: Document the reason (e.g. manual invoicing, test invoices, cancellations). An explainable gap is acceptable in an audit.
- Electronic systems: If you use accounting software, check GoBD compliance: automatic numbering must be traceable, audit-proof and protected against subsequent changes.
- Changing systems: When changing systems you should continue the existing numbering sequence or switch cleanly to new prefixes and document the change.
Conclusion: A well-thought-out, documented invoice numbering system saves time in accounting, reduces audit risks and increases traceability. Choose a simple, consistent system and comply with the requirements of the UStG, AO and GoBD.