Blog · · 4 min read
How to Number Invoices
Invoice numbering looks trivial until an auditor asks why there is no invoice 47. Here is how to pick a system you can live with.
The two rules
Whatever system you choose, numbers must be unique and the sequence must have no gaps. In many countries both are legal requirements; everywhere they are practical ones, because a gap is indistinguishable from a deleted invoice.
Three systems that work
Pick one and never change it mid-year.
- Simple sequential — INV-0001 onward. Best for most solo businesses.
- Year-prefixed — 2026-001, resetting each January. Makes year-end filing easier.
- Client-suffixed — INV-0042-ACME. Keeps the global sequence intact while making the client visible.
What to do about a cancelled invoice
Never delete it and never reuse the number. Issue a credit note against it, or mark it cancelled and keep it in the sequence with a zero value. The gap you avoid creating is worth more than the tidiness you lose.
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