How to number your invoices correctly

Invoice numbering is not a formality. A tax inspector reads it as proof that no invoice has been removed from your books.

Three rules govern it. The sequence must be chronological, it must have no gaps, and a number must never be reused — not even for a cancelled invoice.

The usual format combines a prefix, the year and a counter: INV-2026-0001. The year makes the sequence readable; the counter resets each January, which is allowed as long as the prefix distinguishes the periods.

If you invoice for several companies, give each its own series. Mixing them makes the sequence impossible to audit.

Never delete a cancelled invoice. Issue a credit note that references it: the sequence stays intact and the correction is traceable.