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Key takeaways
- An invoice number is a unique reference that identifies one invoice. Most tax authorities expect a sequential series without unexplained gaps.
- India's GST rules ask for a consecutive serial number of up to 16 characters, unique in the financial year.
- Use a prefix for the entity or document type and a year marker. Never reuse a number, even for a cancelled invoice.
- A proforma invoice, credit note and debit note each get their own series.
Free PDF and Excel: Invoice numbering policy and log. Get it below
What an invoice number does
An invoice number is a unique identifier given to each invoice, so that the seller, the buyer and the tax authority can refer to the same document without doubt.
On the buyer's side it is the key that stops duplicate payments. Accounts payable systems flag a second invoice with the same number from the same supplier. On the seller's side it is the audit trail. An inspector can ask for the full series and expect to account for every number.
A small agency numbers invoices by hand in a spreadsheet. In March it reuses INV-0042 because the earlier one was cancelled. The client's system sees a duplicate and rejects the new invoice as already paid. Payment stalls for three weeks.
The agency moves to a numbering sheet with a rule: a cancelled number stays in the log marked void, and the next invoice takes the next number.
Common formats
| Format | Example | Good for |
|---|---|---|
| Plain sequence | 1042 | Very small businesses |
| Prefix and sequence | INV-1042 | Most businesses |
| Year and sequence | 2026-0042 | Clean annual resets and audits |
| Entity, year, sequence | IN-BLR-26-0042 | Groups with several branches or registrations |
| Customer based | ACME-2026-07 | Retainers and recurring billing |
Free PDF and Excel
Invoice numbering policy and log
A one-page numbering policy, a log that records every number including voids, and a monthly gap check.
- Numbering policy page
- Number log layout
- Monthly gap check
- Excel number generator with year and prefix
Rules worth following
- Number invoices in strict sequence, with no gaps. If you void one, keep it in the log.
- Never reuse a number, even from a cancelled invoice.
- Keep credit notes, debit notes and proforma invoices in separate series.
- Where each branch has its own registration, give each branch its own series.
- Keep the length within local limits. India caps it at 16 characters and allows letters, numbers and the slash and hyphen symbols.
- Reset to 1 each financial year only if your system includes the year in the number.
For the tax-invoice rule behind this see the GST tax invoice page. For the document that sits before the invoice see proforma invoice.
Fields the invoice number should work with
| Field | What goes in it | Why it matters |
|---|---|---|
| Invoice date | Date of issue. | Duplicate detection often matches number and date together. |
| PO number | The purchase order the invoice relates to. | Lets accounts payable match the invoice automatically. |
| Supplier tax number | Registration or tax number. | Duplicates are checked per supplier. |
| Amount | Invoice total. | Duplicate rules often check number, supplier and amount. |
Where numbering breaks
- Two people issuing invoices from two spreadsheets.
- Restarting numbers in the middle of the year.
- Editing a sent invoice and keeping the same number.
- Using customer names in the number, which leaks who you bill.
- Numbering the credit note with the same series as the invoice.
- One person or one system controls the series.
- Voided numbers are kept and marked.
- No numbers are reused.
- Credit notes have their own series.
- Format fits local length rules.
- The log is reviewed each month for gaps.
Last checked: 19 October 2026. Indian 16-character rule checked against published GST guidance. Other countries have their own invoice number rules. Ask your accountant.
Frequently asked questions
Does an invoice number have to be sequential?
Many tax authorities expect it, including India for GST invoices. Even where it is not required, sequential numbering makes audits easier.
Can I use letters in an invoice number?
Yes. A prefix such as INV is common. Check any local limits on length and characters.
What do I do with a cancelled invoice?
Keep the number in your log, mark it void and issue a new invoice with the next number. Do not reuse the cancelled one.
Should a credit note share the invoice number series?
No. Credit notes should have their own sequence, such as CN-0001.
How long should an invoice number be?
Short enough to type and quote. In India the limit is 16 characters.