Guide · Invoicing
Invoice numbering that survives an audit
An invoice number is how you, the client and an auditor find the same document. A simple, consistent sequence is worth more than a clever one.
Why the number matters
The number is the unique key for the invoice. It is what the client puts on the payment, what your bank statement shows and what an auditor uses to trace a sale. If two invoices share a number, or a number is reused after a correction, the trail breaks.
Three systems that work
- Plain sequential: 0001, 0002, 0003. Simple, but it resets awkwardly across years or entities.
- Year prefix: 2026-0001, 2026-0002. Easy to sort and archive; make the year part of the sequence, not a separate counter.
- Client or project prefix: ACME-001, ACME-002. Useful when a client needs its own reference; keep a central log so numbers never collide.
| Example | Good for | Notes |
|---|---|---|
| BC-0001 | A single business, simple archive | Keep the prefix stable |
| 2026-014 | Year-by-year bookkeeping | New year, new sequence |
| ACME-2026-003 | Client-specific references | One central log still required |
Never reuse or edit
Once an invoice has been sent, do not change its number or its amounts. If it is wrong, issue a credit note for the correction and, if needed, a new invoice with a new number. The original stays in the file exactly as it was sent.
Gaps are not automatically a problem
A missing number in the sequence is not fraud by itself — an invoice can be voided, a job cancelled or a draft discarded. What matters is that you can explain the gap if asked. Keep a log with a status for each number: issued, voided, credited.
Separate sequences
Use different prefixes or sequences for invoices, quotes, proformas, credit notes, purchase orders and work orders. If a proforma and an invoice share a number, a client’s AP team cannot tell which document is the bill.
What AP teams and auditors look for
- A unique number on every invoice and credit note.
- Numbers that match the amounts and dates in the ledger.
- A credit note that references the original invoice it corrects.
- No unexplained edits to issued documents.
- A log that explains any gap in the sequence.
When to start a new sequence
Start a new sequence when the year changes, when you start a new legal entity, or when you move to a new accounting system and cannot carry the old numbers forward cleanly. Document the change: “Invoices from 2027 use 2027-0001.” Do not restart mid-year just because the numbers look untidy; continuity is more valuable than a round number.
A simple log
A spreadsheet with number, date, client, amount, status and payment date is enough. If you use accounting software, it keeps the log for you — but check that the sequence has no duplicates when you migrate or import.
The blank invoice template leaves the number blank for you to fill in; the credit note generator uses its own CN- sequence. See invoice record keeping for how long to keep the trail.
Related
Invoice record keeping
What to keep, how long, and how to survive an audit or dispute.
How to write an invoice
The seven parts of a US invoice, a worked example and common mistakes.
When a client will not pay
Written demand, payment plan, small claims and collections.