How to number invoices without painting yourself into a corner

The rule

Invoice numbers must be unique and sequential, with no gaps in the sequence. HMRC cares about this because a gap looks like a deleted invoice, and a deleted invoice looks like undeclared income. You are allowed to cancel an invoice, but the number has to stay in the sequence with a record of the cancellation.

Schemes that work

Anything that is sequential and unambiguous. In practice three shapes cover almost everyone.

• Plain sequential: 1001, 1002, 1003. Simplest, and impossible to get wrong.
• Year-prefixed: 2026-001. Makes filing obvious and resets neatly, as long as the year part never disappears.
• Customer-prefixed: ACME-014. Useful if you are invoicing a handful of large accounts who each want their own run.

Schemes that bite you later

Anything built on a date alone breaks the moment you issue two invoices on the same day. Anything built on the job number breaks when a job needs a second invoice. And starting at 1 tells every customer they are your first, which is a small thing but a real one, so most people start at 1001.

If you have already made a mess of it

Do not renumber history. Pick a number safely above everything you have ever issued, start there, and carry on cleanly from that point. A one-off jump in the sequence is easy to explain; a rewritten back catalogue is not.