How to number invoices correctly

Last reviewed 2026-11-04 · Guidance, not tax or legal advice

Short answer: Give every invoice a unique number that follows a sequence, and never reuse or skip numbers without a record. In India, GST invoice numbers can be up to 16 characters (letters, digits, hyphen and slash) and must be consecutive and unique within a financial year. UK VAT invoices need a unique sequential number. A simple format like INV-2026-0001 works almost everywhere.

Formats that work

Rules by country

• India (GST): up to 16 characters, using letters, digits, - and /; consecutive serial numbers, unique for the financial year. The GST invoice generator checks this.
• UK (VAT): a unique number in a sequence. Gaps should be explainable, for example spoiled or cancelled invoices.
• Australia: tax invoices do not legally need a number, but the ATO and your customers expect one for record keeping.

Tips

• Increase numbers automatically. The generators suggest the next number when you click “Start next invoice”.
• Never reuse a number, even for a cancelled invoice. Issue a credit note instead.
• Use the same number on the PDF file name for easy searching.

Free tools for this

• GST invoice generator (checks the 16-character rule)
• How to make an invoice

Frequently asked questions

More guides

• GST invoice format: every mandatory field, explained
• CGST + SGST or IGST: which one applies to your invoice?
• Tax invoice requirements in Australia
• What a UK VAT invoice must include
• Commercial invoice vs proforma invoice