What an invoice number looks like
There is no single required format. Common patterns include:
- INV-0042: a prefix and a simple counter
- 2026-017: the year and a counter
- ACME-003: a client prefix and a counter
Rules worth following
Whatever format you pick, a few rules keep your records clean.
- Unique: no two invoices share a number
- Sequential: each number follows on from the one before
- Permanent: never changed after the invoice is sent, and never reused
Where it appears
The number sits in the invoice header. Your customer quotes it when they pay, a credit note refers back to it, and a receipt cites it, so one number ties the whole transaction together.
Invoice number or PO number?
The invoice number is yours. A purchase order (PO) number is the customer's reference for their own approval. If the customer gave you a PO number, put it on the invoice as well as your own number.
Frequently Asked Questions
Do invoice numbers have to be sequential?
Many tax systems expect them to be unique and ordered, so sequential numbering is the safe approach. Check the guidance for your country.
Can I change an invoice number after sending the invoice?
Avoid it. Your customer has already referred to the number. If the invoice is wrong, cancel it with a credit note and issue a new one.
What if I skip a number?
Write down why, for example a cancelled draft. A gap you can explain is very different from one you cannot.
Create an invoice with the right terms
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