There's no legally mandated invoice numbering format in most places — the actual requirement is just that each invoice has a number, and it's unique. But an unstructured, inconsistent numbering habit ("invoice1", "invoice-final", "invoice-final-v2") makes bookkeeping and searching a genuine mess later. A few common conventions solve that cleanly.
Sequential
The simplest option: INV-0001, INV-0002, and so on, incrementing by one every time. Easy to implement, easy to spot a gap in (if INV-0047 is missing, you'll notice), and works well for most small businesses issuing a moderate, steady volume of invoices.
Date-based
INV-2026-0001, resetting the counter at the start of each year (or month, for higher-volume businesses) — INV-2026-06-0001 for June. This makes it immediately obvious when an invoice was issued just from its number, and keeps individual counters from growing unwieldy over many years of operation. The trade-off is slightly more logic to implement correctly (remembering to reset the counter at each period boundary).
Client-based
ACME-0012 — a per-client prefix plus a counter that's specific to that client relationship. Useful for businesses that want to see at a glance how many invoices a given client has received, or that organize their records primarily by client rather than chronologically. Less common for very small operations, more common for agencies or consultancies with a manageable, recurring client list.
The rule that actually matters
Whichever convention you pick, never reuse a number, even for a cancelled or voided invoice. If invoice INV-0047 gets cancelled before payment, it stays retired — the next invoice is INV-0048, not another INV-0047. Reusing numbers is what actually breaks bookkeeping and creates confusion during an audit or tax filing, far more than which convention you originally chose.
Using the tool
Our Invoice Generator has a free-text invoice number field, so whichever convention you settle on — sequential, date-based, client-based, or your own — you're not locked into a format the tool imposes on you.
Try it here: Invoice Generator.