Invoice number format best practices for Indian businesses
Rule 46(b) of the CGST Rules is oddly specific about invoice numbers: 16-character limit, unique per financial year, consecutive, alphanumeric with only three special characters permitted. Miss any of these and your buyer can lose ITC. Here's the format we use, and why.
What the law says
"A consecutive serial number not exceeding sixteen characters, in one or multiple series, containing alphabets or numerals or special characters — hyphen or dash and slash symbolised as '-' and '/' respectively, and any combination thereof, unique for a financial year."
— Rule 46(b), CGST Rules 2017
Key constraints:
- Max 16 characters
- Alphabets + numbers + hyphen (
-) + slash (/) only - Consecutive (no skipping, no duplicates)
- Unique per financial year (reset on 1st April)
- You can have multiple series (e.g., branch-wise) but each must be internally consecutive
The format we recommend
INV/YY-YY/NNN — for example INV/25-26/001
Why this works:
- Prefix (
INV/) — instantly identifies it as an invoice (as opposed to a proformaPI/…or credit noteCN/…) - Financial year (
25-26) — makes the series year-obvious even if you export to Excel and sort alphabetically - Three-digit counter (
001) — enough for 999 invoices per year (upgrade to 4-digit0001if you cross 1,000) - Total 13 characters — well within the 16-char limit
Multiple series — when to use them
You can maintain parallel series for different scenarios. Examples:
INV/…for regular tax invoicesEXP/…for exportsPI/…for proforma invoicesCN/…for credit notesDN/…for debit notesRV/…for receipt vouchers (advance)- Branch-wise:
MUM/…,DEL/…,BLR/…
Each series must be internally consecutive. If you issue an INV/25-26/001, the next tax invoice must be INV/25-26/002 — you cannot skip to 003 just because you liked the number.
Real-world mistakes we've seen
Mistake 1 — Repeating numbers across years
Some businesses use only sequential numbers (1, 2, 3…) without a year prefix. In a 5-year audit, this becomes a nightmare because "Invoice #47" could mean four different documents. Add the year in the format.
Mistake 2 — Skipping numbers "for luck"
We know a jeweller who skips 13 because of superstition. That is technically a violation — the series must be consecutive. If the department audits your books and sees a gap, they may demand copies of the missing invoices.
Mistake 3 — Restarting the series mid-year for "clean books"
A common practice among first-time GST-registered businesses. This creates duplicate numbers in the same financial year — instant ITC risk for your buyer.
Mistake 4 — Using spaces or special characters like #, @, .
Rule 46(b) permits only alphanumeric plus - and /. An invoice numbered Invoice #42 or INV.001 is technically non-compliant.
Mistake 5 — Same series across GSTINs
If you have two GSTINs (say, one in Maharashtra and one in Karnataka), each needs its own independent series. You cannot share.
Format examples for different business types
- Freelancer:
AN25-26/001(using initials + year + counter) - Small trader:
ST/2526/001 - Multi-branch:
MH/INV/2526/001andKA/INV/2526/001 - SaaS company:
INV-25-26-00001(5-digit counter for high volume)
Setting up the series in our generator
Our invoice generator auto-suggests the next number based on your last invoice. On 1st April, it prompts you to reset the series. If you want to override, just type your desired number in the "Invoice #" field.