Invoice number series
Every invoice takes its number from a sequence that tax rules expect to be unique and in order, usually restarting each financial year.
Updated October 2026 · with examples from our own builds
What does invoice number series mean in practice?
Indian GST rules expect invoice numbers to be consecutive and unique within a financial year, so a business can show that none are missing or duplicated. That sounds trivial until two invoices are issued at the same moment, an invoice fails halfway, or a new financial year begins.
Each edge case becomes a question for your accountant: a failed invoice that burned a number, a race that issued the same number twice, a first invoice of the year numbered zero.
Sound designs reserve the number atomically at the last moment, only after the invoice has passed validation, and keep separate series for credit notes and for each business entity or branch.
Invoice number series at a glance
- Expected
- Unique and consecutive within each financial year
- Reserve the number
- Atomically, after validation
- Separate series for
- Credit notes, entities and branches
Where has invoice number series come up in our builds?
Each financial year has one counter document, and numbers such as INV/FY26-27/00042 are reserved with an atomic increment only after the invoice validates, so a failed invoice can't burn a number. Our own audit found the first invoice of every financial year had been numbered 00000; it is fixed, and credit notes have their own series.
A document takes the financial year of the moment it is sent, not drafted, and the number is taken atomically inside the send step, so a draft never holds one. The design's rule is that a gap is acceptable and a duplicate never is.
Issuing a business invoice takes the next gapless serial in the financial-year series and freezes the amounts and both parties' GST details; voiding an invoice releases its shipments for re-billing.
Common questions about invoice number series
Can two invoices ever get the same number?
Not if the number is reserved with a single atomic operation and a unique index backs it up. Without both, two requests at the same moment can collide.
What should happen to the number when an invoice fails?
Ideally nothing: validate the invoice fully before reserving a number, so failures never consume one. Storafleet and AskForth reserve only after validation for exactly this reason.
Related terms
Planning a product and not sure where invoice number series fits? Ask us, or browse all 116 terms in the glossary.