Invoice numbering

An invoice number is the reference everyone uses for the document. You quote it when chasing, your client puts it on the remittance, and your accountant uses it to match a payment in the bank to a line in the books. It gets referred to far more often than it gets read. What matters is that it's unique, in order, and short enough to say on the phone.

Sequential, and nothing else

Use a plain increasing number. That can be 1, 2, 3, or 1001, 1002, 1003 if you'd rather not announce that this is your first invoice. You can prefix the year if you like: 2026-001.

The reason is that you can tell whether a sequential number is missing just by looking. Bookkeeping systems, accountants and audits all assume a sequence, because a gap is the easiest way to spot a document that has gone astray. A scheme that encodes other information loses that check.

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. When I recommend keeping to one, that's about what stays easy to check. The law doesn't require it. Most VAT and GST rules elsewhere also ask for a sequential number used only once, and the invoice requirements checker shows which countries require it.

Why client codes go wrong

The tempting scheme is one that carries meaning, such as ACME-2026-004 or 2026-DESIGN-11. It reads well, sorts by client in a folder and looks organised. However, it causes three problems that usually don't show up in the first year.

• One sequence becomes many. You get one sequence per client, so you can't tell by looking whether anything is missing. Say ACME-004 is followed by ACME-006. That might be an error or a cancelled job, and a year later you won't remember which.
• Clients change name. They get acquired, rebrand, or become a subsidiary of another company. Then half your invoices to the same company carry one code and half carry another, and the tidy filing you wanted is gone.
• The number tells the reader things. It's printed on a document that goes to someone else. A number showing that this is your 4th job for Acme and your 11th design project tells them about the size and shape of your business, which you may not have meant to share. That's information given away in exchange for a folder that sorts nicely.

To find every invoice for one client, use a filter instead. Put the client name in the file name or in whatever you keep your records in, and leave the number alone.

Gaps, and what to do about one

At some point you'll raise an invoice and then need it not to exist: the job was cancelled, the figure was wrong, or it went to the wrong company. The instinct is to delete it and reuse the number. Don't.

Issue the replacement as the next number, and keep a record of 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's already in their system, I recommend issuing a credit note that cancels it explicitly. Otherwise the invoice disappears from your records, and your books and theirs no longer agree.

A gap with a note explaining it is fine. Reusing a number means the same reference points at two different documents, which is the problem the number is there to prevent.

Practical details

• Pad the digits. Use 001 rather than 1, so the numbers sort correctly in any folder, spreadsheet or email client.
• Restart each year or don't, 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. The 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.