What the rules actually require
In the UK, HMRC requires invoice numbers to be sequential and unique, and a gap in the sequence is something you must be able to explain. The US has no single federal rule, but the same logic applies at audit: a numbering scheme that cannot demonstrate completeness invites the question of what is missing.
The practical test is whether someone who has never seen your books can tell, from the numbers alone, that nothing has been quietly removed.
Formats that work
Plain sequential. INV-0001. The least to go wrong. Pad to at least four digits — three runs
out sooner than people expect, and INV-99 sorting after INV-100 in a folder listing is a small
recurring annoyance.
Year-prefixed. 2026-001 or INV-2026-0042. Makes the filing year obvious and lets you restart
the counter each January. Restarting is fine provided the year is part of the number, because the
full string stays unique.
Client-coded. ACME-0007. Useful if you invoice a handful of clients heavily. The trade-off is
that you no longer have one sequence to check for completeness, so it is worth keeping a global
counter alongside it.
Formats to avoid
Dates alone. 15082026 breaks the moment you issue two invoices in a day, and it is ambiguous
between UK and US readers besides.
Random or non-sequential numbers. These fail the completeness test outright.
Restarting per client without a client code. Two invoices numbered 001 is a duplicate, however you think about it internally.
Never delete, never reuse
This is where most numbering problems start. An invoice has been sent, you spot an error, and the obvious move is to delete it and issue a corrected one with the same number. Do not.
Once an invoice has left your hands it exists in someone else’s records. Deleting it leaves a gap you cannot explain, and reusing the number means two different documents share an identity — including, potentially, in a VAT return you have already filed.
The correct fix is a credit note referencing the original invoice, followed by a fresh invoice with the next number in sequence. Three documents instead of one, and every one of them defensible.
If you have already made a mess
Do not renumber historic invoices to tidy them up — that makes it worse, because it breaks the link to records your clients already hold. Start a clean sequence from today, with a note in your own records explaining where the change happened.
Last reviewed August 2026.