Invoice Numbering Rules Under GST: Series, Prefixes and Financial Year
By Alphabet Technology · · 4 min read
Quick answer
Under GST, invoice numbers must be consecutive, no longer than 16 characters, use only letters, numbers, hyphens (-) and slashes (/), and be unique within a financial year. A common format is PREFIX/YEAR/NUMBER, such as INV/2026-27/001. You may run separate series for branches, proformas and credit notes.
Good formats
| Document | Example |
|---|---|
| Tax invoice | INV/2026-27/001 |
| Proforma | PI/2026-27/001 |
| Credit note | CN/2026-27/001 |
| Export | EXP/2026-27/001 |
| Branch series | JPR/2026-27/001 |
Rules to follow
- Restart or continue the series each 1 April – but never repeat a number within the same year.
- Report the series used in your GSTR-1 document summary.
- Don't skip numbers; if an invoice is cancelled, keep it as cancelled.
Merged invoices
When you combine several bills into one, Invoice24 keeps a combined number such as INV/2026-27/12-13 so the trail stays clear.
Frequently asked questions
Must invoice numbers restart every financial year?
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They must be unique within the financial year. Most businesses restart at 001 each April with the year in the prefix.
Can I use the date in the invoice number?
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Yes, as long as the whole number stays within 16 characters and is unique for the year.
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