Invoice number generator
Pick a shape, set where you are up to, and take the next number. The point of a format is not the format — it is that every invoice you ever send follows the same one.
How invoice numbering works
An invoice number has one job: to identify one document, permanently and uniquely. Three rules cover almost every situation. Never reuse a number. Never leave a gap you cannot explain. Never change the number on an invoice you have already sent.
If you cancel an invoice, the usual practice is to void it — keeping the number, marked cancelled — and issue the replacement with the next number. Deleting the document leaves a hole that someone will ask about a year later.
Resetting the counter each January is fine as long as the year is part of the number, which is why INV-2026-001 is a common default. Sequences per client work too, but only if you keep a note of where each client is up to.
Record-keeping requirements for invoice numbering vary by country. This page describes common practice, not a legal rule where you are.
Let it number itself
Keeping a counter in your head works until the day it does not. In a free Invoira account the number is handed out by the database at the moment you issue an invoice, one caller at a time — so drafts never use a number up, and two open tabs cannot produce the same one.