In one line
The client's own reference for an order, quoted on your invoice so their accounts team can match it.
Also called a purchase order number, a customer PO, or an order reference.
What it means in practice
Larger clients raise a purchase order before work starts and will not pay an invoice that does not carry its number. It is the single most common reason an invoice sits in someone's system untouched.
Ask for the PO number when the job is agreed, not when the invoice bounces. Put it near your own invoice number, clearly labelled as theirs.
Covered in full in the guide to invoice numbers.
Example. "Your PO: 4520" beside "Invoice: INV-2026-0041".
Where it shows up on the paperwork
It goes at the top of your invoice next to your own invoice number, clearly labelled as theirs. Some clients also want it on the delivery note and the packing slip.
What goes wrong
- Sending the invoice and asking for the PO number afterwards.
- Putting their PO number where your invoice number should be, so neither system matches.
- Guessing at a number rather than asking, which produces an invoice that will bounce quietly.
The tools for this
Related terms
- Purchase orderA document you issue to a supplier that commits to buying named goods at named prices.
- Invoice numberThe unique reference that identifies one invoice, running in sequence with no repeats.
- Remittance adviceThe note a client sends saying which invoices a payment covers.
Common questions
What happens if I invoice without a PO number?
At a large client, usually nothing at all: the invoice sits unmatched until someone chases it. Ask for the number when the job is agreed, not when the invoice is late.
Is a PO number the same as an invoice number?
No. The PO number is the client's reference for the order they raised. The invoice number is yours for the bill you issued. Both appear on the invoice.