What is a purchase order number (PO number)?
By Diljit Ramachandran, founder of The Invoice Generator. Published Oct 26, 2026.
A PO number is the reference number of a purchase order, the document a buyer sends to approve a purchase. Printing it on your invoice lets the client match your bill to their order, which is often required before they will pay.
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Where do you get the PO number?
The buyer gives it to you, on the purchase order or in the email that approves the work. You do not make it up. Wait for the PO before you start the work, then invoice against it. A purchase order is the buyer's written order; the difference from an invoice is who sends it and when. A purchase order generator makes one. Writing your own? See how to make a purchase order.
What does a PO number look like?
Letters and digits the buyer sets, such as PO-2026-0412. Copy it exactly.
Where does it go on the invoice?
Follow these steps:
- Add a field labelled PO number near the invoice number and date
- Copy the digits exactly
- Check the client's invoicing instructions; some want it in the email subject too
What if the PO number is missing or the amount differs?
A missing PO number often gets the invoice held or returned, and payment slips. A three-way match compares the PO, the delivery and the invoice.
| Situation | What to do |
|---|---|
| No PO number | Ask the client before you send |
| Amount differs | Ask for an amended PO or change order before the extra work, then invoice against it |
Questions
- Is a PO number the same as an invoice number?
- No. The PO number is the buyer's reference for the order. The invoice number is your own reference for the bill.
- What does PO invoice mean?
- An invoice that refers to a purchase order number. A purchase order generator makes the buyer's side.
Keep every document in one place
- A free account saves a copy of each invoice, remembers your clients and items, and shows what is paid and overdue.
- Pro numbers your invoices for you, emails payment reminders, and repeats the ones you send every month.
Drafted with AI assistance from public sources and our own tool, and published by Diljit Ramachandran. It has not been checked by an accountant. It is general information, not tax or legal advice. Rules differ by place; check with an accountant or your local tax office.