Invoice Number Format: Examples and Best Practices
Every invoice needs a unique number. In the EU, VAT rules require invoice numbers to follow one or more sequences that identify each invoice uniquely. Even where it isn't required, consistent numbering makes bookkeeping and payment tracking much easier.
Common formats
- Simple sequence: 001, 002, 003
- Year + sequence: 2026-001, 2026-002 (restart each year)
- Prefix + year + sequence: INV-2026-001
- Client code + sequence: ACME-001 (combine with a global sequence to stay unique)
Best practices
- Never reuse or skip numbers without a reason you can explain.
- Don't delete an invoice that was sent; issue a credit note instead.
- Use leading zeros (001) so numbers sort correctly.
- Pick one format and stick to it.
Credit notes
To correct an invoice, issue a credit note with its own number (for example CN-2026-001) that refers to the original invoice number.
Billowly remembers your last invoice details in your browser, so you only need to increase the number for the next invoice.