The core requirement
GST requires invoice numbers to be sequential and unique within a financial year, for each series you use — no gaps you can't explain, no repeats, and no numbers reused after being cancelled.
Beyond that, you have real flexibility: alphanumeric prefixes, separate series per branch or document type, and a reset at the start of each financial year are all allowed.
What actually breaks under review
The two habits that draw questions: numbers that jump around without a consistent series (making it look like invoices are missing), and manually renumbering after the fact to close a gap — which looks far worse than the gap itself.
A cancelled or voided invoice number should stay retired, clearly marked, rather than being reissued to a different sale.
Resetting your numbering mid-year — outside the financial-year boundary — is one of the fastest ways to turn a clean series into an audit flag.
LedgrBook handles this automatically. The parts of this guide your bills already know.
See how →Setting it up once, properly
The easiest way to stay compliant is to decide your numbering format per document type and financial year up front, then let the system allocate numbers automatically at save — removing the temptation to hand-edit a sequence later.
General information, not professional tax advice — confirm specifics with your CA.
