An invoice number is the one field that has to be unique on every document you send. The format you pick does not matter to tax authorities as much as the rule that no number is ever reused.
Pick one unbroken series and never reuse, skip or edit a number. Sequential (INV-001), year-prefixed (2026-001), year-month (2026-08-014) and client-based (ACME-003) all work; the format matters far less than the discipline. In the EU and the UK a sequential number that uniquely identifies the invoice is a legal requirement, based on one or more series. In the United States there is no federal rule.
| Format | Example | Best for |
|---|---|---|
| Sequential | INV-001, INV-002 | Low volume, first invoices ever sent |
| Year-prefixed | 2026-001, 2026-002 | Anyone who wants the year visible at a glance |
| Year-month | 2026-07-014 | Higher volume, monthly bookkeeping |
| Client-based | ACME-003 | Few clients billed often, easy filing per client |
Whichever you pick, the number goes straight into the invoice header and becomes part of the downloaded PDF file name, so keep it short enough to read at a glance.
Once you have picked a scheme, put it straight to use in the invoice generator or the quote generator.
Every legal requirement on this page comes from the authority that publishes it. Dates below are the day we last opened each page.
In the EU, yes. The VAT Directive requires a sequential number, based on one or more series, that uniquely identifies the invoice, and a UK VAT invoice needs the same. That is a legal requirement, not a convention. In the United States there is no federal rule about invoice numbering at all, though an unbroken series is still standard practice and makes your own records defensible. Note what the EU rule actually allows: more than one series is fine, so a per-client or per-year prefix is permitted as long as each series runs without gaps.
Yes, and this is exactly what the EU rule's phrase based on one or more series permits: a year-prefixed number such as 2027-001 is its own series and never collides with 2026-001. Restarting a bare counter with no year in it would reuse numbers, which is the thing to avoid. Keep the full number unique and the reset is fine.
Do not edit or delete it, because that breaks the unbroken series your records depend on. Issue a credit note that refers to the original invoice number and cancels it in full or in part, then raise a new invoice with the next number in your series. Under the EU VAT Directive, a document that amends and refers specifically and unambiguously to the initial invoice is treated as an invoice in its own right.
It appears in the header band of the invoice or quote, next to the totals, and it is used to name the downloaded PDF file, for example invoice-INV-2026-014.pdf.
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