Invoice numbering

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-20261001-002` sorts before `-010`; `INV-20261001-2` sorts after `-10`.
• Never reused, including for a cancelled invoice. Void it and let the number stay spent.

Schemes that work

• `INV-20261001-001` — the day, and which one of that day it is. It sorts by date, says when without anybody looking it up, and needs no count carried from one invoice to the next. This is how Perly numbers every document, a repeating invoice's copies included.
• `INV-0001` — one ascending series forever. Simple and correct, as long as one place keeps the count for you.
• `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, a bill — what you owe — is best filed under the supplier's number, because that is the number they will chase you with. In Perly a new bill is offered a number for its day, after the bill prefix in Settings — `BILL-20260930-001`; type the supplier's number over it and the bill keeps theirs. Either way invoices and bills are counted apart, so what you owe never takes a number from your own invoices. 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 reserves nothing: it suggests the day and which document of the day it is — `INV-20261001-001` — and saves whatever number the document holds when you save it, one you have used before included. A blank you opened and closed without saving leaves nothing behind. A repeating invoice is numbered the same way: each copy for the day it is made, one past the highest number that day already has, one you typed by hand included. Deleting a saved document or an unsent copy can leave a gap, because the count goes on from the day's highest number and never goes back to fill one.

If it is already a mess

1. Stop. Do not renumber anything that has been sent — those documents exist in somebody else's books.
2. Write down which series exist and where each one has got to.
3. Pick one, start it above the highest number you have ever issued, and use only that from now on.
4. Keep a note of what happened and when you changed. An explained discontinuity is an administrative event; an unexplained one is a finding.

The free invoice generator numbers a new sheet for the day, like INV-20261101-001, and lets you type over it with whatever series you already keep.