Blog · October 25, 2026 · 3 min read

Invoice Numbering: Simple Systems That Keep You Organised

Every invoice needs its own number. A good system makes invoices easy to find, helps you spot a missing one, and avoids confusion when a customer asks about "invoice 12".

Common formats

• Simple sequence: 1001, 1002, 1003.
• Year and sequence: INV-2026-00001, INV-2026-00002. The year makes old invoices easy to place. This is the format the Lumara invoice generator suggests by default.
• Customer prefix: ACME-001, ACME-002, if you invoice a few large customers separately.

Tips

• Never reuse a number. If an invoice is wrong, issue a corrected one or a credit note rather than reusing the number.
• Cancel instead of deleting. Marking an invoice as Cancelled keeps the sequence clear for your records.
• Pick one system and stay with it. Consistency matters more than the format.
• Check local rules. Some countries require an unbroken sequence. Ask your accountant if you are unsure.

How the invoice generator helps

The invoice generator suggests the next number from the invoices saved in your browser, lets you edit it, and refuses to save a number that already exists among your saved invoices. You can change the prefix, the number of digits and whether the year is included under Backup and settings on the My Invoices page. It can only check invoices saved in the same browser, so keep your own list if you also invoice from other devices.

More guides

• How to Create a Professional Invoice in 5 Steps
• What to Include on an Invoice: A Practical Checklist
• VAT, Sales Tax and Discounts: How Invoice Totals Are Calculated
• How to Save an Invoice as a PDF on A4 or Letter Paper