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Billing & invoicing

How to number GST invoices: the rules, and how to never break them

Consecutive, unique in the financial year, at most 16 characters — the GST rules for invoice numbers, and how two counters avoid clashing.

Billyng team · Published · 1 min read

Invoice numbers look like a detail until two invoices share one. Then they are an audit question.

What the rules ask

Rule 46 of the CGST Rules says a tax invoice carries a serial number that is:

  • Consecutive — no gaps you cannot explain.
  • At most 16 characters, using letters, numbers, hyphens or dashes, and slashes.
  • Unique for the financial year — no number used twice in one year.

You may run several series

The rule allows one or multiple series. That is useful: a shop with two counters can give each its own series — A/1001, B/1001 — and both stay consecutive and unique. What you must not do is let two counters draw from one series without coordinating, because they will eventually issue the same number.

A good format

  • A short prefix that says what it is — INV for invoices, QT for quotations.
  • A counter that only goes up.
  • Optionally the financial year, if you restart numbering every April — INV/26-27/0001.

Keep it well inside 16 characters, so the year or a branch code can be added later without breaking the limit.

The two mistakes to avoid

  1. Typing numbers by hand. Someone will skip one or repeat one.
  2. One series on two machines that cannot talk to each other — the classic offline clash.

In Billyng

Billyng numbers every bill for you, and quotations in their own series. Each kind of machine bills from its own series — the desktop’s (INVD-) and the web app’s (INVW-) — so a counter working offline and a colleague billing in the browser can never issue the same number. Numbers start at 1001 and only go up.

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