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.
| 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.
They have to be unique and never reused, but the format is up to you. Purely sequential (INV-001, INV-002) is the simplest to keep unique. Prefixes and year codes just make a number easier to scan later.
Yes, many people do, for example resetting to 2027-001 each January. As long as the full number (including the year) is never reused, restarting the counter is fine.
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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