Guide
Invoice number format: examples and rules
Pick a numbering format you can keep for years. Here are the common ones, what tax authorities actually require, and how to fix a gap or a mistake.
Common invoice number formats
Any format works if every invoice gets a number nobody else in your records has, and the numbers follow an order you can explain. These are the formats small businesses use most. The last column shows what Next № in Billpad gives after each one.
| Format | Example | Good for | Watch out for | Next № |
|---|---|---|---|---|
| Plain sequence | 0042 | Very small businesses | Shows clients how few invoices you send; start at 0001, not 1, so numbers sort correctly | 0043 |
| Prefix + sequence | INV-0042 | Keeping invoices apart from quotes (EST-) and receipts (RCT-) | Keep the same number of digits so files sort in order | INV-0043 |
| Year + sequence | INV-2026-0042 | Finding a year’s invoices at a glance; restarting each year | Restart only at the start of a year or financial year, never mid-year | INV-2026-0043 |
| Year + month | 2026-10-007 | Monthly billing cycles | The count is still one series: don’t reuse 001 every month unless each month is its own declared series | 2026-10-008 |
| Client prefix | ACME-007 | A few large clients who want their own count | Several series to keep gap-free; check that separate series are allowed where you are registered | ACME-008 |
| Project code | P112-03 | Stage payments on long projects | You still need one overall sequence for your records | P112-04 |
Credit notes usually get their own series (for example CN-0001) and refer to the number of the invoice they correct.
What the rules say about sequences
Where a tax authority sets rules on invoice numbers, they are about the same thing: each number must identify one invoice, and the numbers must follow on so that a missing invoice stands out. These are the rules we could confirm in official sources. The invoice requirements by country page lists every other required item.
- United Kingdom. A VAT invoice needs a unique number that follows on from the number of the previous invoice (HMRC: VAT Notice 700/21 (record keeping)).
- European Union. The VAT Directive requires a sequential number, based on one or more series, that identifies the invoice uniquely (Council Directive 2006/112/EC (VAT Directive), Article 226(2)). Separate series are allowed; each member state can add detail.
- India. A GST tax invoice needs a consecutive serial number of no more than 16 characters, in one or more series, unique for the financial year. Letters, numbers, the hyphen “-” and the slash “/” are allowed (CBIC: CGST Rules 2017, rule 46 (tax invoice)).
- United States, Canada and Australia. The official guidance we checked does not list an invoice number among the required items. Number your invoices anyway: clients’ accounts teams, auditors and your own bookkeeping all rely on it.
Gaps, duplicates and mistakes
- Never delete an invoice you have sent. If it is wrong, issue a credit note against it and a new invoice with the next number, so the sequence still accounts for every number.
- A skipped number (you jumped from 0042 to 0044): note in your records why 0043 does not exist, for example “not issued, number skipped in error”.
- A duplicate number sent to two clients: correct the later one with a credit note and reissue it with the next free number.
- Switching format (say, to year-based numbers): switch at the start of a year, and keep a note of the last number in the old format.
Numbering in Billpad
- Type your first number in the number field at the top of the invoice, in any format up to 40 characters.
- Each time you download or print, Billpad remembers that number. Next № (next to the number) and New invoice move to the following one and keep the zero padding: INV-009 becomes INV-010, 2026-099 becomes 2026-100.
- Invoices, proformas, estimates and receipts each keep their own count, so EST-0019 never pushes your invoice numbers forward.
- Saved invoices in the settings panel lists every number you downloaded or printed. Download list (CSV) gives you a register to check for gaps.
The count is kept in this browser. If you invoice from two devices, move your data with Back up or move your data in the settings panel (the backup includes the numbering), so both continue the same sequence.
Updated 28 October 2026. General information, not legal or tax advice. Spotted an error or an outdated rule? Contact us. About Billpad.