Invoice numbering
An invoice number is the only handle anybody has on the document — your client's accounts payable, your accountant, your own memory in eighteen months. It has one job, which is to be unique and to be findable.
The rules worth keeping
- Unique. One number, one document, forever. This is the only rule with no exceptions.
- Sequential and ascending. It should be obvious that nothing is missing between two invoices.
- No gaps you cannot explain. In several jurisdictions a gap in a VAT-registered series is a question an inspector is entitled to ask.
- One shape, kept. Fix the format and the padding at the start. `INV-0001` sorts correctly at the thousandth invoice; `INV-1` does not.
- Never reused, including for a cancelled invoice. Void it and let the number stay spent.
Schemes that work
- `INV-0001` — one ascending series forever. The simplest thing that is correct, and the right default.
- `2026-014` — restarting each year. Tidy, common, and what many accountants ask for; just never let two years collide.
- `2026-03-014` — year and month. Useful if you invoice a lot and search by period.
- `ACME-014` — per client. Avoid it: parallel series are how the same number ends up issued twice, and your client does not need their own name in it.
Whatever you pick, your bills — what you owe — keep the supplier's numbering, not yours, because that is the number they will chase you with. A separate series for each direction is what stops your own from developing holes. See invoice or bill.
When the number is taken matters
Most tools reserve a number the moment you open a blank invoice, which means every draft you abandoned is a hole in the series you will one day have to explain. Perly takes the number at save, not at open: a document you started and closed leaves nothing behind. A repeating invoice takes the next number when its copy is written, for the same reason.
If it is already a mess
- Stop. Do not renumber anything that has been sent — those documents exist in somebody else's books.
- Write down which series exist and where each one has got to.
- Pick one, start it above the highest number you have ever issued, and use only that from now on.
- Keep a note of what happened and when you changed. An explained discontinuity is an administrative event; an unexplained one is a finding.
Common questions
- Can invoice numbers contain letters?
- Yes. A prefix is normal and useful. Keep it constant and put the varying part at the end.
- Do I have to start at 1?
- No, and plenty of people do not, to avoid announcing that this is their first invoice. Starting at 100 is fine; skipping about afterwards is not.
- What do I do about a cancelled invoice?
- Void it and keep the number used. Struck through in your records, never deleted and never reissued under the same number.
- Can two clients have the same invoice number?
- No. Uniqueness is across your whole business, not per client.
- How do I correct an invoice that has already gone out?
- A credit note or a corrected invoice, each with its own new number, referencing the original. Never an edited copy of a document somebody already has.
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