Invoice numbering rules for Canadian small businesses

A number should be unique, sequential and readable at a glance. A common pattern is a prefix, the year, then a zero-padded counter — for example INV-2026-0001. The prefix keeps document types apart, the year makes annual resets safe, and the padding keeps everything sorting correctly in a spreadsheet.

• Unique across your whole history, not just this year
• Assigned in order, with no gaps you cannot explain
• Never reused, even when an invoice is cancelled
• Separate sequences for invoices, estimates, credit notes and receipts

A gap is not automatically a problem — it is only a problem if you cannot explain it. The risk is that two people create an invoice at the same moment and both take the same number, or that a cancelled invoice is deleted rather than credited.

The correct fix for a wrong invoice is a credit note that references the original, then a new invoice with the corrected details. Both documents stay in your records, and the trail from one to the other is what makes the correction defensible.

Numbers are assigned by the database at the moment of issue, with a row-level lock on the sequence, so two people issuing at once can never take the same number. Invoices, estimates, credit notes and receipts each have their own sequence, with an optional annual reset.

Once an invoice is issued, its number and financial fields are frozen. Corrections go through credit notes, so your sequence stays intact.

Does the CRA require sequential invoice numbers?

There is no single prescribed format, but every invoice must be uniquely identifiable and your records must be complete and traceable. Sequential numbering is the simplest way to show nothing is missing.

Can I delete an invoice I sent by mistake?

No. Issue a credit note that references the original invoice number, then issue a corrected invoice. Deleting leaves a gap you cannot explain later.

Can I restart numbering each year?

Yes, as long as the year is part of the number so it stays unique — for example INV-2026-0001. Restarting a bare counter creates duplicates across years.

What about estimates and credit notes?

Keep them on their own sequences with their own prefixes. Mixing them into the invoice sequence makes it look like invoices are missing.