GST invoice numbering: the 16-character rule that quietly breaks spreadsheets
A GST invoice number is more constrained than most people realise: sixteen characters, a restricted alphabet, and consecutive within the financial year. Most homegrown schemes break at least one of those.
Rule 46(b) of the CGST Rules requires 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."
Unpacked, that is four separate constraints.
- Maximum sixteen characters. Not sixteen digits — sixteen characters including any prefix and separators.
- Only letters, digits, hyphen and slash. No spaces, no underscores, no hash, no dots.
- Consecutive. Gaps invite questions about missing invoices; do not skip numbers you dislike.
- Unique for a financial year, which in India runs April to March.
A scheme that fits
The pattern that survives contact with reality is a short prefix, a two-digit financial year pair, and a zero-padded sequence:
| Pattern | Example | Length | Verdict |
|---|---|---|---|
| PREFIX/FY/NNNN | INV/26-27/0001 | 14 | Fits, readable, sortable |
| PREFIX/FY/NNNNN | AS/26-27/00001 | 14 | Fits, room for 99,999 invoices |
| PREFIX-YYYY-NNNN | ACME-2026-0001 | 14 | Fits, but ambiguous across a financial year |
| PREFIX/YYYY-YYYY/NNNN | INV/2026-2027/0001 | 18 | Too long |
Note the third row. Using a calendar year is legal but unhelpful, because the financial year straddles two calendar years — an invoice dated February 2027 belongs to FY 26-27, and a number reading 2027 will confuse whoever reconciles it.
Multiple series are allowed
The rule explicitly permits "one or multiple series". If you need separate numbering for branches, product lines or export versus domestic sales, use distinct prefixes: DOM/26-27/0001 and EXP/26-27/0001 can coexist. Each series must be consecutive within itself.
Consecutive means no gaps
If you cancel an invoice, do not silently reuse or skip the number. Issue a credit note against it, or mark it cancelled and keep the number in the series. A gap in the sequence is the first thing an officer looks for, and "we deleted it" is a much worse answer than "here is the credit note".
How Invoicr handles it
The India GST format treats a number that breaks Rule 46(b) as an error rather than a warning: over sixteen characters, or containing a disallowed character, and the builder tells you before you send. It also offers a compliant suggestion derived from the invoice date, so the financial-year label is always the right one, and increments the sequence for you when you save with an account.
Invoicr validates structure and arithmetic, not your business facts. This is general information, not tax, accounting or legal advice — check anything consequential with your accountant.