Document types

Purchase order (PO)

A purchase order (PO) is a buyer-issued document that authorises a supplier to provide specific goods or services at agreed prices and terms. The PO number is the unique identifier the buyer's procurement system gives that document (for example 4500098231 or PO-2026-00123), and it is the string the supplier quotes on the invoice so accounts payable can match the bill to the order before paying it.

Applies in: Global

The PO is the buyer's side of the transaction. Procurement or a budget owner raises it before any work happens, which authorises the spend and creates a number the supplier quotes on every invoice that follows. When the invoice arrives, accounts payable matches it against the PO and, in most enterprises, against the goods receipt or approved timesheet. That is the three-way match.

If the PO number is missing or wrong, the invoice gets held. AP has nothing to match it to, the spend is not authorised against anything in their system, and the invoice sits in an exception queue until a human chases it. Suppliers are routinely surprised by how long that delays payment on work everyone agreed to weeks earlier.

What is a PO number on an invoice?

The PO number on an invoice is the unique identifier the buyer's procurement system assigned to the purchase order, reproduced on the invoice as a reference. It is not the purchase order itself. The purchase order is the document (line items, quantities, agreed prices, an authorised total, an approver); the PO number is the short string, usually 6 to 12 characters, that AP keys in to pull that document up and check your invoice against it. Suppliers never invent it. You copy it, character for character, from the buyer's PO confirmation.

What it looks like depends on which system issued it:

Issuing systemTypical patternExample
SAPusually 10 digits, commonly starting 454500098231
Oracle EBS, PeopleSoft10 digits, no prefix1000482913
Coupaoften a letter prefix followed by digitsA1043771
NetSuite, small-buyer spreadsheetPO prefix plus a running sequencePO-12345
Buyer-defined with a year segmentPrefix, year, zero-padded sequencePO-2026-00123
Large retailRetailer prefix plus 6 to 12 digitsWM4471820

Paste it, do not retype it. To an AP system, 4500098231, PO 4500-098231, and 4500 098231 are three different strings, and only one of them matches an open PO. If you are not sure whether what the buyer sent is a real PO number or a placeholder someone typed during quoting, run it through the PO number format checker.

Where the PO number goes on an invoice

Position on the invoiceField label you will seeWhat goes in it
Header block, beside the invoice number and issue datePO number, Purchase order no., Customer PO, Your referenceThe PO string exactly as the buyer wrote it, on its own labelled line. This is the one that matters: OCR capture and AP scanning read the header first.
Line-item table (enterprise buyers with itemised POs)PO line, ItemThe PO line number each charge maps to. SAP numbers PO lines in tens, so the first line is 00010.
AP portal upload or email subjectPO, Invoice referenceThe same string again. Coupa and Tungsten-style portals reject a submission with the PO field left blank.
No PO issued for this workThe same PO fieldWords, not a blank: "No PO required per Jane Okafor, 14 May 2026". A populated field routes better than an empty one.

What does not work is burying the PO number in the payment-terms footer or in the body of the covering email. AP processes the document, not the email.

Worked example: one invoice against PO 4500098231

Northwind Retail raises PO 4500098231 on 4 May 2026: line 00010, UX consultancy, 40 hours at GBP 95.00, authorised total GBP 3,800.00 excluding VAT, terms Net 30. You deliver 38 hours and bill on 31 May.

Your invoice header carries two numbers, clearly labelled and never merged: Invoice number INV-2026-0412 (yours, from your own sequence) and PO number 4500098231 (theirs, copied from the PO). The single line reads "UX consultancy, sprint 4 (PO line 00010), 38 hrs @ GBP 95.00 = GBP 3,610.00". VAT at 20% adds GBP 722.00, so GBP 4,332.00 is due by 30 June 2026.

AP then runs the match:

Leg of the matchWhat AP comparesResult
1. Find the POInvoice PO 4500098231 against open POs in the ledgerOne open PO found
2. Invoice to POLine 00010 description, unit price GBP 95.00, invoice net GBP 3,610.00 against the GBP 3,800.00 ceilingUnder the ceiling, prices agree
3. ReceiptApproved timesheet for 38 hoursQuantity agrees

Three passes and the invoice joins the next payment run without a person opening it. Bill 44 hours instead (GBP 4,180.00 net) and leg 2 fails: you are GBP 380.00, or 10%, over the authorised total, and many AP teams allow only a few percent of over-tolerance. The invoice parks until Northwind raises a PO amendment. Under-billing never fails the match; over-billing does.

Whether you get a PO at all is set by the buyer's spend controls, not by the size of your invoice. Enterprise and public-sector buyers run committed spend through a PO as a rule; mid-size buyers often require one only above a threshold (GBP 1,000 and USD 5,000 are common); many owner-run businesses never issue one. Ask at kickoff: do you raise a PO, and what number should the invoice quote. That question takes 30 seconds and saves the two-week exception queue.

If the number only lands after you have invoiced, you do not have to reissue. JupiterInvoice treats the PO as a recipient-editable field, so whoever opens the invoice link can type it in themselves, versioned and with you notified. The full process, from quote to three-way match, is in the purchase order guide.

Common questions about Purchase order

What is a PO number on an invoice?
It is the buyer's purchase order identifier, quoted on your invoice so their accounts payable team can match the bill to the order they authorised. The buyer's procurement system generates it (SAP issues 10 digits commonly starting 45, such as 4500098231; smaller buyers use formats like PO-2026-00123), the supplier never invents it, and it belongs in the invoice header next to your own invoice number.
Where do I put the PO number on an invoice?
In the header block, next to the invoice number and issue date, on its own line labelled "PO number" (buyers also use "Purchase order no.", "Customer PO", or "Your reference"). Put it above the line-item table, not in a footer note: AP capture software reads the header first. If the buyer's PO is itemised, also quote the PO line number against the matching invoice line, for example "PO line 00010".
Is a PO number the same as an invoice number?
No. The PO number is the buyer's reference, generated by their procurement system before the work starts (4500098231). The invoice number is your reference, generated by your own sequence when you bill (INV-2026-0412). Both belong on the invoice, separately labelled: AP uses the PO number to match the charge to an authorised order, and their remittance advice quotes your invoice number back to you. One PO can also cover many invoices, which is exactly why the two cannot share a field.
What if the buyer has not given me a PO number yet?
Ask the person who commissioned the work rather than AP: the requester raised the requisition and can see the number in their system. If it will not arrive before you need to bill, put something in the PO field ("PO to follow" or "No PO required per Jane Okafor, 14 May 2026") rather than leaving it blank, then add the real number when it lands. On JupiterInvoice the recipient can add or correct the PO on the invoice link itself, so you amend rather than reissue.
Do I need a PO to send an invoice?
Not always. Smaller buyers, especially those not running a formal procurement system, will pay invoices without a PO. Larger and public-sector buyers usually require one and will hold an invoice that arrives without it. The right move is to ask before the work starts: do you raise a PO, and if so, what number should I put on the invoice.
What happens if I send an invoice without a PO number?
At a typical enterprise, the invoice gets parked in an exception queue. AP cannot match it without the PO, so they email the requester or the supplier to find the PO. Resolution time is often measured in weeks, not days. The simplest fix is to add or amend the PO number on the invoice itself once you have it, rather than sending a replacement invoice.
Who issues a purchase order?
The buyer. PO issuance is a buyer-side control to make sure spend is authorised before it happens. The supplier never issues a PO; the supplier issues the invoice. If a buyer asks you to send a PO, they almost certainly mean a quote or a proforma invoice.

Last updated August 23, 2026

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