What is a Purchase Order (PO) Number and Why Do You Need It?

What's a PO Number?

A Purchase Order (PO) is an official request from a buyer to a supplier. The PO number is a unique identifier the buyer assigns to track their purchase. If a buyer requests a PO number, you must include it on your invoice or payment may be delayed or withheld.

Who Requires PO Numbers?

• Large corporations — Almost always require PO numbers
• Government/public sector — Mandatory. Invoice won't process without it
• Small businesses — Rarely require POs
• Consumers — Don't use PO numbers

Example Invoice with PO Number

Invoice Number: INV-0089
PO Reference: PO-2026-4567
Date: 15 May 2026

[Rest of invoice details...]

What If Customer Doesn't Give You a PO?

Ask them for it before starting work. Don't invoice without it if it's a corporate client — payment will be rejected.

Invoice Processing: PO Matching

Large companies match invoices to POs automatically. If you quote PO-2026-4567 but invoice shows PO-2026-4568, the invoice gets rejected by their system.

FAQ

What if the PO is for a different amount than the invoice?

Contact the buyer immediately. Don't invoice more than the PO authorises — it will be rejected.

Can I invoice without a PO?

If they don't ask for one, no. But if they're a large company, always request it first.