What an invoice number has to do
An invoice number has to meet two conditions: it appears only once, and it is assigned sequentially. That is what § 14 (4) sentence 1 no. 4 UStG says: a sequential number with one or more series that is assigned once to identify the invoice. What the number looks like is up to you. A format such as RE-2026-0001 meets both conditions and stays readable after years.
The number is one of the mandatory details on every invoice. If it is missing or used twice, your customer can run into trouble with the input tax deduction until you correct the invoice. The other details are in the guide to invoice requirements.
Allowed formats with examples
Under section 14.5 (10) UStAE the tax authorities allow one or more series of numbers or letters and any combination of digits and letters. These formats are common:
| Format | Example | When it fits |
|---|---|---|
| plain number | 1001, 1002, 1003 | few invoices, one counter forever |
| year and counter | 2026-001 | the counter restarts every year |
| prefix, year and counter | RE-2026-0001 | to tell invoices from other documents |
| year, month and counter | 2026-10-017 | many invoices per month |
| prefix per area | RK-2026-0001 | own range for invoice corrections |
If the counter restarts every year, the year must be part of the number. Otherwise 001 would come back every year and the number would no longer be unique. The same applies to a counter that restarts every month.
If you are just starting out, you don’t have to begin at 1. A starting number such as 1001 or 2026-100 is allowed. Many people like it because the first invoice then doesn’t look like the first.
What to keep in mind for the format
A good format is short, sorts itself and stays the same for years. Leading zeros as in 0007 keep your invoices in the right order in every list and folder. Plan enough digits: with three digits you run out at 999 invoices a year.
Don’t build in details that can change, such as a customer’s name or a project code that won’t exist next year. Think of your customer too: they type the number into the reference of their bank transfer. The shorter and clearer it is, the easier you match the payment later.
Several number ranges
You may keep several number ranges side by side, for example per year, per location or per document type (section 14.5 (10) and (11) UStAE). All that matters is that each invoice can be assigned clearly to one range and no number appears twice. A prefix such as RE for invoices and RK for invoice corrections makes this clear at a glance.
A cancellation invoice or an invoice correction also gets its own number. It refers to the original invoice but does not replace its number.
Gaps, drafts and the turn of the year
Is a gap forbidden?
No. Under section 14.5 (10) UStAE a gapless sequence is not mandatory. VAT law requires uniqueness, not a perfect chain. Your bookkeeping is a little stricter. Records must be complete and orderly (§ 146 (1) AO). If a number is missing, the tax office may ask during an audit whether income is missing. If further inconsistencies come up, it may estimate (§ 162 AO).
So write down why a number is missing, for example “skipped when switching programs”. More on orderly bookkeeping is in the guide to GoBD.
Drafts without a number
Most gaps appear when a number is assigned as soon as an invoice is created and the draft then ends up in the trash. If you assign the number only when the invoice is done and goes out, this gap never appears. You never delete an invoice you sent; you cancel it with a cancellation invoice. That way its number stays used.
At the turn of the year
Whether you restart every year or simply keep counting is your choice. Both are allowed. If you change the format, you keep the old numbers unchanged and start the new series so that no number can be created twice.
When no number is needed
For a small-amount invoice up to €250 gross the invoice number is not mandatory (§ 33 UStDV). Since 2025 small businesses also need no number on the invoice under § 34a UStDV. It still pays off in both cases: you find every invoice faster, your customer can quote it when paying and your tax advisor sees at a glance whether everything is there.
This guide explains the rules in general. How to switch an existing number series cleanly is a question for your tax advisor if in doubt.
Common questions
May I include the customer number in the invoice number?
Yes, as long as the final number stays unique and can be assigned clearly to one number range. With many customers this quickly gets confusing, though. One central counter with the year is usually simpler, and the customer number then appears separately on the invoice.
Does the invoice number have to start at 1?
No. You may start with any number, such as 1001 or 2026-100. All that matters is that it continues sequentially and no number appears twice.
What do I do if I used a number twice?
Cancel one of the two invoices, usually the one your customer has not recorded yet, and issue it again with a new number. Send your customer the new invoice with a short note. How this works is in the guide How to correct an invoice.
May I change my number format?
Yes, for example at the turn of the year. The numbers already issued stay as they are. Just make sure the new series cannot create a number that already exists.