Invoice numbering

An invoice number has one job: to be the unique handle everybody uses when referring to this document. You use it when chasing. Your client uses it on the remittance. Your accountant uses it to tie a payment in the bank to a line in the books. It is referenced far more often than it is read, which is why the qualities that matter are boring ones — unique, ordered, short enough to say on the phone.

Sequential, and nothing else

The scheme to use is a plain increasing integer: 1, 2, 3, or 1001, 1002, 1003 if you would rather not announce that this is your first invoice. Optionally prefix the year: 2026-001. That is the whole recommendation.

It is boring on purpose. A sequential number is the only scheme where the question "is one missing?" has an answer you can get by looking. Every bookkeeping system, every accountant and every audit assumes it, because a gap in a sequence is the cheapest possible signal that a document has gone astray. Schemes that encode information are schemes that give that signal up.

In the UK a sequence is also the rule for VAT invoices: HMRC's VAT Notice 700, section 16.3.1, requires "a sequential number based on one or more series which uniquely identifies the document". Note the "one or more" — several series are allowed. The case below for keeping to one is advice about what stays easy to check, not a legal requirement.

Why client codes go wrong

The tempting scheme is one that carries meaning: ACME-2026-004, or 2026-DESIGN-11. It reads well, it sorts by client in a folder, and it feels organised. It causes three problems, none of which show up in the first year.

The first is that you now have one sequence per client rather than one sequence. Nobody can tell by inspection whether anything is missing, because ACME-004 followed by ACME-006 might be an error or might be a job that was cancelled — and you will not remember which.

The second is that clients change name. They get acquired, they rebrand, they become a subsidiary of something else. Now half your invoices to the same company carry one code and half carry another, and the tidiness the scheme was for is gone.

The third is the quiet one: the number is on the document, and the document leaves your building. A number encoding that this is your fourth job for Acme and your eleventh design project tells the reader things about the size and shape of your business that you did not decide to tell them. It is not a disaster. It is simply information you gave away for free in exchange for a folder that sorts nicely.

If you want to find every invoice for one client, that is a filter, not a numbering scheme. Put the client name in the file name or in whatever you keep the records in, and leave the number alone.

Gaps, and what to do about one

At some point you will raise an invoice and then need it not to exist — the job was cancelled, the figure was wrong, it went to the wrong company. The instinct is to delete it and reuse the number. Do not.

Issue the replacement as the next number in the sequence, and keep a record of what happened to the original. If the wrong invoice never reached the client, a note to yourself is enough. If it did reach them — and particularly if it has already been entered in their system — the clean move is a credit note that cancels it explicitly, rather than a quiet disappearance that leaves two accounting systems disagreeing.

A sequence with a documented gap is a sequence that still works. A sequence where numbers are reused is one where the same reference points at two different documents, which is precisely the failure the number existed to prevent.

Practical details

• Pad the digits. 001 rather than 1, so the numbers sort correctly in any folder, spreadsheet or email client without anybody having to think about it.
• Restart at the year or do not — but pick one. 2026-001 restarting each January is fine. A single sequence running forever is also fine. Switching halfway is what creates the mess.
• Keep it short enough to read aloud. This number gets spoken on calls and typed into payment references. Every extra segment is another chance for it to be typed wrong on a remittance you then have to reconcile by hand.
• Avoid characters that travel badly. Slashes and spaces get mangled in file names, URLs and some payment reference fields. A hyphen is safe everywhere.