How to number invoices.
Every invoice needs a unique number so you, your client and the tax authority can identify it. The rules are simple: numbers should be unique, sequential and never reused. Here are the common numbering systems, with examples, so you can pick one and stay consistent.
The rules of invoice numbering
Unique
No two invoices share a number — ever.
Sequential
Numbers should follow an unbroken order with no gaps.
No reuse
Never recycle a number, even after a cancellation.
Consistent
Pick one system and stick to it across all clients.
Common numbering systems
Simple sequential
INV-001, INV-002, INV-003 — easy and clear.
Year-based
2026-001, 2026-002 — resets neatly each year.
Date-based
20260527-01 uses the date plus a daily counter.
Client-based
ACME-001, ACME-002 to group by customer.
Avoid gaps
If you void one, issue a credit note rather than skipping a number.
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Frequently asked questions
How should I number my invoices?
Use a unique, sequential system such as INV-001, INV-002, or a year-based format like 2026-001. The key rules are that every number is unique, follows an order, and is never reused.
Can invoice numbers have gaps?
It is best to avoid gaps, because they can look like a missing or hidden invoice to an auditor. If you cancel an invoice, issue a credit note against it rather than deleting the number.
Can I start invoice numbers at a higher figure?
Yes — many new businesses start at, say, INV-1000 so early clients don’t see they are the very first customer. Just keep it sequential from there.
Do I need a prefix like "INV"?
A prefix is optional but helpful — it makes invoices easy to spot and lets you separate invoices from quotes or credit notes.
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