An invoice number is a primary key. Your client's accounts system uses it to match a payment to a bill; your own records use it to prove a sale existed; a tax inspector uses it to check nothing was quietly deleted. Those three uses pull in slightly different directions, which is why the question of what to name your invoices is less trivial than it looks.
The three rules that actually matter
- Unique. No number may ever appear on two invoices, even across years, even after a cancellation.
- Sequential. Most bookkeeping regimes expect numbers to run in order with no unexplained gaps, so that a deleted invoice is visible as a hole.
- Stable in format. Changing scheme mid-year makes sorting and searching painful for the rest of the business's life.
Scheme 1: plain sequential
0001, 0002, 0003. Simple, unambiguous, and the easiest to defend. The drawback is social: a client receiving invoice 0003 knows you have billed twice before, which some freelancers would rather not advertise. Starting at 1001 is a common and harmless workaround.
Pad with leading zeros to at least four digits so that text sorting matches numeric sorting.
Scheme 2: year-prefixed sequential
2026-001, 2026-002. The year is instantly readable, files sort themselves chronologically, and the counter resets each January which keeps numbers short. This is the default recommendation for most one-person businesses. Decide now whether the counter restarts each year — restarting is fine as long as the year prefix is always present, because the full string remains unique.
Scheme 3: client-coded
ACME-2026-004. Useful when you bill a handful of clients repeatedly and want to see at a glance which relationship an invoice belongs to. The cost is a per-client counter to maintain and a sequence that is no longer globally continuous, which some accountants dislike. If you use this, keep a global sequence in your own ledger alongside it.
Scheme 4: date-stamped
20260815-01. Guaranteed unique if you never raise more than 99 invoices in a day, and it encodes the issue date. It reads as machine output rather than as a business document, so it suits high-volume automated billing more than client-facing freelance work.
What to avoid
- Random numbers. They break sequence checks and make it impossible to notice a missing invoice.
- Reusing a number after cancelling. Cancel by issuing a credit note that references the original; never recycle the identifier.
- Slashes and spaces.
2026/01is fine on paper but breaks filenames and some banking reference fields. Hyphens are safe everywhere. - Very long strings. Bank transfer reference fields are frequently limited to around 16 to 18 characters. If your number is truncated, reconciliation fails.
Handling corrections
Once an invoice has left your outbox it is a record. If the amount was wrong, do not quietly edit and resend the same number — issue a credit note for the original and a fresh invoice with the next number, and say plainly in the notes that it supersedes the earlier one. If the client has not opened it and nothing has been recorded on their side, a corrected reissue under the same number is usually acceptable, but keep a copy of both versions.
Duplicating a recurring invoice
Retainers and monthly work are the main source of numbering mistakes, because the temptation is to resend last month's PDF with a new date. The safe pattern is to duplicate the record and let the number increment automatically. InvoiceGen's invoice list does exactly this: duplicating an invoice copies every detail, sets today's date, and advances the number to the next unused value, so two live invoices can never share an identifier.
Keeping the register
Whatever scheme you choose, keep one list of every number issued, the client, the date and the amount. A spreadsheet is enough. The register is what lets you answer, in ten seconds, the question an accountant will eventually ask: what happened to invoice 2026-017?