Billing › Invoice number rules

GST invoice number rules

Invoice numbers look like a small detail, but GST returns list every invoice by its number. A repeated or broken series is one of the first things a tax officer notices.

The rules

• Running order: numbers go in sequence, one after another.
• Up to 16 characters, counting letters, numbers and symbols.
• Allowed characters: letters (A–Z), numbers (0–9), hyphen (-) and slash (/). No spaces or other symbols.
• Unique in the financial year: the same number can't appear twice between 1 April and 31 March.
• One or more series: you may keep separate series, for example one per branch or one for exports, as long as each number is still unique.

Good formats

Putting the year in the number (26-27) lets you start again from 001 every April without ever repeating a number.

Things to avoid

• Restarting at 001 in the middle of the year.
• Using the same series in two shops or two computers without a branch code: sooner or later both make "number 57".
• Long prefixes like SHARMA-TRADERS/2026-27/001 (26 characters, too long).
• Skipping numbers without a reason. If a bill is cancelled, keep its number with "Cancelled" instead of reusing it.

How the invoice maker helps

It suggests the next number in the INV/YY-YY/001 pattern for each business and financial year, counting your saved invoices. You can type your own format: it checks the length and characters and warns you before saving a number that already exists.