Invoice numbers: what the rules in Austria and Germany say

In Austria and Germany, an invoice number must be sequential and may be assigned only once. That is what § 11 (1) no. 3 lit. h UStG (AT) and § 14 (4) sentence 1 no. 4 UStG (DE) say. Neither law prescribes a gap-free series, and the German tax administration states expressly that it is not mandatory. Letters, years and several number series are allowed, as long as the system is clear.

What does the law require?

Both countries use almost the same wording:

• Germany: "a sequential number with one or more series of numbers, assigned once by the issuer to identify the invoice" (§ 14 (4) sentence 1 no. 4 UStG).
• Austria: "a sequential number with one or more series of numbers, assigned once to identify the invoice" (§ 11 (1) no. 3 lit. h UStG).

So the number has to be:

1. Unique: no number may exist twice, not even in different years or for different clients. Every invoice must be clearly identifiable.
2. Sequential: the numbers follow a fixed, traceable system. The law prescribes no format, so arbitrary random numbers are risky.

"One or more series" means you may run several number series side by side, for example one per year or per location.

There are exceptions: small-amount invoices need no number (up to €250 in Germany under § 33 UStDV, up to €400 in Austria under § 11 (6) UStG), and in Germany § 34a UStDV doesn't require small businesses under § 19 UStG to number their invoices either. Number them anyway. Your clients and your bookkeeper work with it.

Does the number have to be gap-free?

No, but gaps should be explainable.

Germany. The tax administration states expressly in the VAT application decree that a gap-free sequence of invoice numbers is not mandatory (UStAE 14.5 (10)). What matters is that every number is unique.

Austria. The law requires a sequential number; it contains no express gap-free rule. According to the Austrian VAT guidelines (UStR 2000 paras 1539, 1548–1552), you may choose where numbering starts, as long as it is systematic, and use letters and several invoice series. Your client doesn't have to check whether the numbers are sequential.

Why gaps still matter. The number matters to you too: in a tax audit, a continuous series shows that no invoice is missing. Your records must be complete and orderly (§ 131 BAO in Austria, § 146 AO in Germany). An unexplained gap raises questions.

If a number is skipped, write down why. For example: "2026-017 not used, draft deleted on 12/03/2026." Software that assigns numbers only when an invoice is issued avoids such gaps in the first place.

Which number formats work?

All these formats are common in both countries, as long as every number stays unique:

Good to know:

• Starting value: you don't have to start at 1. 2026-101 is just as valid and doesn't reveal that it is your first invoice.
• New year: with the year in the format, you may restart the count each year, so 2027-001 follows 2026-148. Without the year, keep counting.

With client codes, the combination of code and counter must stay unique, and the system should be defined and written down.

If a contract for an ongoing service is itself the invoice, for example a rental or maintenance contract with monthly payments, a unique number from the contract (for example the property or tenant number) is enough in Germany, and the payment receipts need no number of their own (UStAE 14.5 (12)). Where an e-invoice is required, the number must be in its structured data. If you send a separate invoice every month, each one gets its own number.

Can you use several number series?

Several number series side by side are allowed in both countries, as long as it is clear which invoice belongs to which series. Typical splits:

• by document type: invoices INV-2026-…, cancellation invoices CN-2026-…
• by location or unit: Vienna W-2026-…, Graz G-2026-…
• by period: per year or per month

In Austria, the guidelines mention branches, permanent establishments and cash registers as separate invoice series, and allow a separate series for self-billed invoices (UStR 2000 paras 1549 and 1552). Germany allows separate series for areas separated by time, geography or organisation (UStAE 14.5 (11)).

Quotes are not invoices. They don't need an invoice number and don't belong in the invoice series. Give them their own quote numbers.

What happens to the number when you cancel?

You don't change an invoice after sending it, and that includes its number. Instead:

1. The original invoice keeps its number and stays in your files, for example 2026-031.
2. The cancellation invoice gets a new number (2026-032 or CN-2026-004) and states the number and date of the original invoice.
3. The corrected invoice gets another new number (2026-033).

If a detail is missing or wrong, in Germany you can send a correction document instead of cancelling, as long as it refers specifically and clearly to the original invoice (§ 31 (5) UStDV). It is enough for the document to state the original invoice number; it doesn't need a new number of its own (UStAE 14.11 (1)).

You never reuse a cancelled number.

Can you switch systems in the middle of the year?

Moving from Word, Excel or another program? You have two clean options:

• Keep counting: the last invoice in the old system was 2026-047, the first in the new one is 2026-048. This is the clearest option.
• New series: you start a new series with its own prefix, such as B-2026-001. It must not overlap with the old series.

Note the switch date and the last old number, and keep the old invoices. The new program must never start again at 2026-001 if that number already exists.