How to number invoices, and why the sequence matters

An invoice number is not decoration. It is how you, your client and eventually an auditor agree on which document you are all talking about. Get the scheme right once and it stops being something you think about.

What belongs in a reference

Three parts are enough: who issued it, when, and which one it is. Something like ETH-2026-0001 tells you the issuing company, the year, and that it is the first of that year. Anyone can read it aloud over the phone without spelling anything.

Resist putting the client in the reference. Clients get acquired, rename themselves and merge, and a reference that encodes who they were in 2026 ages badly. Keep the client in the client field, where it can be corrected.

Never reuse a number, and try not to skip one

Two invoices sharing a number is the expensive mistake: payments get applied to the wrong document and the error usually surfaces months later during a reconciliation. Make the sequence the one thing your system guarantees.

Gaps are a softer problem. A missing number is not illegal, but it invites the question “what happened to 0042?” and you want a better answer than a shrug. If you void an invoice, keep the document and mark it void rather than deleting it, so the sequence stays whole and the void is on the record.

Sequential, not random

Random or timestamp-based references are tempting because collisions are unlikely. The trouble is that nobody can tell whether one came before another, and you lose the simplest possible audit signal: that the numbers count up in the order you issued them.

Running more than one company

If you invoice under several names, each needs its own sequence and its own prefix. ETH-2026-0001 and GRO-2026-0001 can exist on the same day without ambiguity, because the prefix says which set of books each belongs to. One shared sequence across companies means your first company’s invoice numbers have holes in them wherever the second company issued one, which is exactly the gap problem above, self-inflicted.

Reset each year, or run forever?

Both work. Resetting to 0001 each January keeps numbers short and makes the year obvious, as long as the year is in the reference. Running forever avoids the question entirely. What matters is picking one and not changing your mind in March.

Reserve the number when you save, not when you send

If the number is only assigned at the moment of sending, two invoices drafted in parallel can end up fighting over it. Assign it when the document is first saved and treat it as spoken for from then on.