Example
INV-2026-0041 sorts correctly, reads out cleanly on the phone and never collides.
Also called an invoice reference, an invoice ID, or a document number.
What it means in practice
Clients pay by invoice number and their software rejects a duplicate, so a number is never reused and a canceled invoice keeps its number and is marked void.
Any format works as long as it sorts and never repeats. A year prefix with a padded count, such as 2026-0041, suits most businesses and shows the year at a glance.
Covered in full in the guide to invoice numbers.
Where it shows up on the paperwork
Top of the invoice next to the date, in the PDF filename, in the email subject, and as the payment reference. Those four places are what make an invoice findable a year later.
What goes wrong
- Reusing a number after a canceled invoice, which the client's software rejects as a duplicate.
- Numbering by date, which collides the first time you send two invoices in one day.
- Renumbering old invoices when you change format, breaking every reference the client already holds.
Common questions
Can I start my invoice numbers anywhere?
Yes. Any starting point works as long as the sequence runs from there without repeats. Starting at 1000 is common and costs nothing.
What if I need to cancel an invoice?
Keep the number and mark it void, then issue the correction with the next number and a line saying which one it replaces. Deleting it leaves a gap you will be asked about.