Why invoice numbers matter
An invoice number lets you, your customer and the tax office find one bill quickly. It also shows that no bill is missing. Tax officers look for gaps or repeats in your numbers, so a clean series protects you.
The basic rules
- Unique: never use the same number twice.
- In order: each new invoice gets the next number.
- No gaps if you can avoid them. If you cancel an invoice, keep the cancelled copy instead of reusing its number.
- Easy to read: a short prefix and a number work best.
GST rules in India
Under Rule 46 of the CGST Rules, a tax invoice needs a consecutive serial number that is unique for the financial year. It can be at most 16 characters long and can use:
- Letters A to Z (capital or small)
- Numbers 0 to 9
- Hyphen "-" and slash "/"
Many businesses add the financial year so numbers can restart each April, for example INV/2026-27/001. You can also keep separate series for different branches or for different documents, such as one for tax invoices and one for delivery challans.
Tip: Count the characters. "SGT/INV/2026-27/0001" is 20 characters and too long for GST. "SGT/26-27/0001" is 14 and fine.
Examples you can copy
| Format | Example | Good for |
|---|---|---|
| Prefix + number | INV-1001 | Simple businesses anywhere |
| Financial year | INV/26-27/001 | GST in India, restarts each April |
| Branch code | AMD/26-27/045 | More than one branch |
| Document type | QT-0012, DC-0034 | Quotations and challans |
| Year and month | 2026-10-017 | Many invoices each month |
UK and other VAT countries
HMRC asks for a unique sequential number on every VAT invoice. Letters and numbers are both allowed. Other VAT countries have similar rules: unique, in order, and easy to trace.