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How to Number Invoices: A System That Stays Clean Forever

Invoice numbers aren't bureaucracy — they're how you (and your client's accounting department, and the tax authority) track money. Here's a numbering system that stays clean forever.

Choose a format and stick to it

Pick one format and never deviate:

• Simple sequential: INV-001, INV-002, INV-003…
• Year-prefixed: 2026-001, 2026-002… (resets each year; most popular with accountants)
• Client-prefixed: ACME-001 for client-specific sequences

Keep estimates in a separate sequence (EST-001…) so the two never collide. Billia does this automatically.

The 3 rules of invoice numbering

Three rules that keep you out of trouble:

1. Never reuse a number. Even for a corrected invoice — issue a new number and void the old one.
2. Never go backwards. Skipped numbers are fine; duplicated or out-of-order numbers look like fraud to an auditor.
3. Keep voided invoices. If you cancel INV-014, keep the record marked 'VOID'. Tax authorities (especially for VAT) may ask about gaps in the sequence.

What the law requires

VAT-registered businesses in the UK and EU must use sequential numbering with no unexplained gaps. Australia and most other jurisdictions require 'unique' numbers — sequential is the simplest way to guarantee uniqueness.

The practical takeaway: sequential, gapless-as-possible, one sequence per document type. Do that and you're compliant essentially everywhere.