What does the law say about invoice numbers?
Rule 46(b) of the CGST Rules asks for "a consecutive serial number not exceeding sixteen characters, in one or multiple series, containing alphabets or numerals or special characters", the special characters being the hyphen or dash and the slash, "unique for a financial year". Credit notes and debit notes follow the same rule (Rule 53).
In plain terms:
- At most 16 characters, counting every letter, digit and symbol.
- Letters, digits, - and / only. No spaces, underscores, hashes or dots.
- Consecutive within a series: 000123 is followed by 000124.
- Unique in the financial year (1 April to 31 March) for your GSTIN.
- One or more series. Several series are allowed, each consecutive on its own.
Start the number with a letter or a digit, not a symbol.
Formats that work, and formats that fail
| Number | Length | Valid? |
|---|---|---|
| INV/26-27/000123 | 16 | Yes, exactly at the limit |
| INV/26-27/0123 | 14 | Yes |
| 2627/000123 | 11 | Yes |
| SHOP-INV-2026-27-000123 | 23 | No, too long |
| INV 2026 000123 | 15 | No, spaces |
| INV_000123 | 10 | No, underscore |
| #1043 (Shopify's order number) | 5 | No, a hash, and order numbers are not invoice numbers |
Why one long number can stop a whole return
The GST portal checks every document number in a GSTR-1 file before it processes a single row. One number over 16 characters rejects the entire upload, and the error does not say which invoice caused it. By then the numbers are issued and cannot be changed, so the series has to be right from the first invoice. Choose a prefix that leaves room: "INV/26-27/" is 10 characters, leaving 6 for the running number, which is enough for 9,99,999 invoices in a year.
Should invoice numbers restart every April?
You may restart, or continue one series that never repeats; either keeps every number unique in its financial year. Most stores restart each April with the year in the prefix (INV/26-27/, then INV/27-28/), so a number shows its year at a glance and the running part stays short.
Can I use more than one series?
Yes. Common reasons:
- Document types: INV/ for invoices, CN/ for credit notes, DN/ for debit notes.
- Channels: one series for the Shopify store, another for a marketplace or a shop counter.
- Registrations: each GSTIN has its own series, because each is its own registration.
Each series is reported as its own range in Table 13 of GSTR-1.
What if I switch invoice apps in the middle of the year?
Continue the series; do not start again from 1. A second INV/26-27/000001 in the same year is a duplicate. Note the last number the old app issued and start the new app from the next one, for invoices, credit notes and debit notes alike.
What if a number is skipped?
A gap is not an offence, but it has to be explained. A number that was issued and then cancelled before it carried any sale is reported as cancelled in Table 13. A number never issued at all should be rare; the cleanest series have none.
Is the Shopify order number an invoice number?
No. Shopify's order number (#1043) is an order reference: it starts with a hash, can be renamed, and also counts orders that are never invoiced. Keep it on the invoice for reference, next to the invoice's own number.