Guides

How to number invoices so you never have to explain a gap

A scheme that stays sequential, sorts correctly, survives a year boundary, and does not advertise how few invoices you have sent.

Numbering looks trivial until an accountant asks why 2027-014 is missing and you have no answer. Several jurisdictions expect an unbroken sequence; all of them expect uniqueness. The scheme below satisfies both and takes no thought once set up.

A scheme that works

Use YYYY-NNN, restarting the counter each January: 2026-001, 2026-002. It sorts chronologically in any file listing, it is obviously unique, and the year is visible without opening the document.

• Never reuse a number, even for an invoice you cancelled. Issue a credit note against it instead.
• Never leave a gap. If you void one, the number stays void and documented — a missing number looks like a hidden invoice.
• Pad the counter (001, not 1) so sorting stays correct past number 9.
• You may start above 001. Beginning a first year at 2026-100 is common and harmless; what matters is that it never goes backwards.

Per-client prefixes

Prefixing by client (ACME-2026-001) reads nicely and creates several parallel sequences, which is exactly what a strict sequential requirement does not want. If you need the client visible, put it in a separate reference field, not in the number.

Credit notes

A credit note cancels or reduces an invoice that was already issued. Give it its own sequence (CN-2026-001) and reference the original invoice number on it. Editing an issued invoice and resending it under the same number is the thing to avoid — the client may have booked the first version already.

Worth knowing

• This generator keeps your counter in your browser and advances it when you start a new invoice, so the sequence holds as long as you use the same browser. Export your data if you change machines.