Invoice numbers look trivial until a gap, a repeat or a 17-character number shows up in your GSTR-1. Here are the rules, and a numbering scheme that stays clean year after year.
What the law says
- The number must be consecutive.
- It can have at most 16 characters.
- Allowed: letters, numbers, hyphen "-" and slash "/". Other symbols are not allowed.
- It must be unique for the financial year.
- You can run one or more series, for example per branch or per document type.
A practical numbering scheme
| Document | Example | Length |
|---|---|---|
| Tax invoice | INV/26-27/0058 | 14 |
| Quotation | QUO/26-27/0012 | 14 |
| Proforma | EST/26-27/0004 | 14 |
| Credit note | CN/26-27/0003 | 13 |
| Debit note | DN/26-27/0001 | 13 |
| Delivery challan | DC/26-27/0021 | 13 |
| Composition bill | CMP/26-27/0101 | 14 |
Putting the financial year in the number (26-27) makes every year unique without any risk, and a separate prefix for each type keeps credit notes from mixing with invoices.
Starting a new series on 1 April
Because numbers must be unique only within a financial year, most businesses restart from 0001 on 1 April. With the year in the prefix, the new series is automatically different from last year's. Good billing software does this reset for you.
Cancelled invoices and gaps
Never delete an invoice and reuse its number. Cancel it instead: the number stays used and is reported as cancelled in GSTR-1 Table 13 (Documents issued). This explains every gap in your series to an officer.
Table 13 of GSTR-1
Table 13 lists, for each document type, the first and last number issued in the period, the total and the number cancelled. Software that tracks your series fills it automatically.
Mistakes to avoid
- Numbers longer than 16 characters, such as long branch codes.
- Manual numbering in Excel, which leads to duplicates when two people bill at once.
- Using the same series for invoices and credit notes.
- Restarting numbers mid-year.
- Special characters like #, _ or spaces.
📌 GST law changes often. This guide reflects the rules as we understand them in October 2026. Confirm anything specific to your business with your CA or on gst.gov.in.
Software for this: NexoraGST GST Billing Software · for small business · in Delhi
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