Core concepts
Purchase order number
A purchase order number (PO number) is the unique identifier assigned to a purchase order, used to track the order and match it to the invoice that follows.
What a PO number is
When a buyer issues a purchase order to authorize a purchase, that order gets a unique PO number. It acts as a reference code that follows the transaction from order to delivery to payment.
The buyer typically asks the seller to put the PO number on the resulting invoice, so the invoice can be matched back to the approved order.
Why PO numbers matter
PO numbers are the backbone of "PO matching" — the process accounts-payable teams use to confirm that an invoice corresponds to an authorized order before paying it. An invoice without the requested PO number often gets held up.
For sellers, adding the buyer’s PO number to your invoice is one of the simplest ways to get paid faster by larger clients with formal purchasing processes.
Example: A buyer issues PO #PO-2026-0142 to a supplier. The supplier delivers the goods and references "PO-2026-0142" on the invoice, so the buyer’s AP team can match and approve it quickly.
FAQs
Frequently asked questions
Where do I put the PO number on an invoice?
Place the PO number near the top of the invoice, alongside the invoice number and date, so the buyer’s accounts-payable team can match it to the order immediately.
What happens if an invoice is missing the PO number?
Invoices without the requested PO number are often delayed, because the buyer’s AP team can’t match them to an authorized purchase order before payment.
Related terms
Purchase order
A purchase order (PO) is a document a buyer sends to a seller to officially authorize and commit to a purchase, listing the items, quantities, and agreed prices.
Invoice number
An invoice number is a unique identifier assigned to each invoice so it can be tracked, referenced, and reconciled in accounting records.
Invoice
An invoice is a document a seller sends to a buyer that itemizes goods or services provided and requests payment by a stated due date.
Put it into practice
WaffleInvoice lets you create branded invoices, set payment terms, collect payments online, and automate reminders — free for unlimited invoices.
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