GST invoice numbering
Invoice numbers are how you, your customer and the tax department track each sale. GST sets a few simple rules.
By Aura Labs · Last reviewed September 2026
The rules
- Consecutive: numbers follow in sequence without gaps
- Unique for the financial year (April to March)
- Up to 16 characters
- Letters, digits, hyphen (-) and slash (/) only
Good formats
| Format | Example | Length |
|---|---|---|
| Prefix + number | INV-0042 | 8 |
| Prefix + year + number | INV/26-27/042 | 13 |
| Branch + year + number | PUN/2627/0042 | 13 |
Mistakes to avoid
- Using dates or random numbers that don't run in sequence
- Formats longer than 16 characters
- Reusing a number after deleting an invoice
- Maintaining numbers by hand in two places
GST rates and rules change. Please verify current rules with official government sources or your CA before filing.
Frequently asked questions
They must be unique within a financial year. Most businesses start a fresh series each April, often with the year in the prefix, such as INV/26-27/001.
Yes — for example separate series for branches or for export invoices — as long as each is consecutive and unique.
Don't reuse the number. Keep the cancelled invoice on record, or issue a credit note if it was already reported.
Official sources
Automatic numbering
Set a prefix and starting number in BillBook and each new invoice takes the next number.
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