Formats that work in the real world
Readable beats clever. Prefer a short prefix, zero-padded digits, and a rule you can explain in one sentence.
- INV-0001 — prefix + padded sequence
- 2026-014 — year + sequence within the year
- STUDIO-220 — brand prefix + continuous counter
- CN-0003 — separate sequence for credit notes
Rules worth keeping
Never reuse a number. Prefer sequential issues. Do not renumber after payment or filing. Estimates and quotes can use EST-/QTE- prefixes so nobody treats them as tax invoices — see document types.
On this site, type the next number into the invoice field before PDF. Keep a simple note of the last number issued. Full workflow: how to create an invoice.
Gaps, credit notes, and multi-entity habits
If you void a sent invoice, keep the number and issue a credit note rather than deleting history. Separate legal entities should not share one continuous counter unless your accountant designs it that way.
Self-employed habits that pair well with numbering: self-employed invoice.
Where the number appears
Put the same number on the PDF, in the filename, and in the invoice email subject so AP can match payment to the bill.
Field checklist: what to include on an invoice.
Disclaimer
Some tax authorities have specific sequencing rules. Confirm requirements for your country; this guide is general practice for clear client billing.