How to number invoices

An invoice number is the primary key of your bookkeeping. Tax authorities in many countries require numbers to be unique and issued in sequence, and any gap can invite questions about invoices you never filed.

Formats that work

• Sequential — 0001, 0002, 0003. Simple, always unique, easy to spot gaps.
• Date plus sequence — INV-2026-0142. Sorts naturally and shows the year at a glance.
• Customer prefixed — ACME-2026-014. Useful when several freelancers bill the same buyer.
• Year reset — INV-2026-0001 restarting each January, which most accountants expect.

Rules to keep it clean

• Never reuse a number, even for a cancelled invoice — issue a credit note instead.
• Do not leave gaps; if you delete a draft, do it before it receives a number.
• Use a consistent prefix so the number never collides with a purchase order reference.
• Record the number on credit notes and receipts so they link back to the original.

A practical scheme

Pick one pattern, write it down, and let software assign the next value automatically. INV-YYYY-NNNN with a four-digit counter covers almost every small business: it is unique across years, it sorts correctly, and a human can tell at a glance whether invoice 0141 came before 0142.