What Is a PO Number on an Invoice? (And Why Missing It Delays Payment)
A purchase order number links your invoice to the buyer's approved purchase. Learn what it is, where to put it, and what to do if a client doesn't give you one.
The short answer
A PO (purchase order) number is a reference created by the buyer when they approve a purchase. When you put that number on your invoice, the buyer's accounts-payable team can match your invoice to the approved order and pay it. Without it, many organisations cannot process the invoice at all.
How purchase orders work
- The buyer decides to purchase goods or services from you.
- Their procurement system issues a purchase order with a unique PO number, listing what is being bought, quantities, prices and terms.
- You deliver the goods or complete the work.
- You send an invoice quoting the PO number.
- Accounts payable matches the invoice to the PO (and often to a delivery receipt) and approves payment.
This matching process (often called two-way or three-way matching) is how larger organisations control spending. If your invoice doesn't match the PO, it is held until someone resolves the difference.
Where to put the PO number
Put it near the top of the invoice, close to the invoice number and dates, labelled clearly: "PO number: 4500012345". Many templates have a dedicated field for it. Our invoice generator includes a PO number field that appears in the invoice header and on the PDF.
If one invoice covers several purchase orders, list the PO number on each line item instead.
Invoice number vs PO number
| Invoice number | PO number | |
|---|---|---|
| Created by | Seller (you) | Buyer (your client) |
| Created when | When you bill | When the purchase is approved |
| Purpose | Identify your invoice | Identify the buyer's approved order |
| Required? | Always | Only if the buyer uses purchase orders |
Read more about creating your own numbering scheme in invoice numbering.
Making your invoice match the PO
- Use the same item descriptions as the PO wherever possible.
- Bill the same quantities and unit prices. If something changed, ask the client to amend the PO before invoicing.
- Don't invoice more than the PO value. The excess will usually be rejected.
- Bill the legal entity named on the PO, not the person who emailed you.
- Send the invoice to the address the PO specifies, which is often a central accounts-payable inbox.
What if the client doesn't give you a PO number?
Ask at the start of the project: "Will you be issuing a purchase order for this work?" Small businesses and individuals usually don't use POs, and that's fine: leave the field blank. But if a larger organisation hasn't given you one, it often means the purchase hasn't been approved in their system yet. Chasing the PO before you start work is far easier than chasing payment afterwards.
Common PO problems
- Expired PO. Some POs cover a fixed period or amount. For long projects, check that the PO still has funds before invoicing.
- Typos. One wrong digit means no match. Copy and paste the number rather than retyping it.
- Multiple POs. Never combine work from different POs into one line.
Missing references are one of the most frequent reasons invoices are rejected; see common invoice mistakes for the others.
Frequently asked questions
Is a PO number legally required on an invoice?
Generally, no. Tax and invoicing laws rarely require a purchase order number, so it is usually a contractual or procedural requirement set by the buyer. In practice, that can be just as important: many larger organizations will not pay an invoice without a valid PO number. If your client uses purchase orders, treat the PO number as mandatory for that client.
What does a PO number usually look like?
There is no standard format. PO numbers are generated by the buyer's procurement or accounting system, so they may be purely numeric, include letters, or carry a prefix such as PO- or a department code. Copy the number exactly as it appears on the purchase order, including any leading zeros or prefixes, because a small difference can prevent automatic matching.
What is the difference between a purchase order and a quote?
A quote is your offer to the buyer, showing what you will supply and at what price. A purchase order is the buyer's formal acceptance and commitment to buy, usually raised after they approve your quote. When the PO is issued, check that its items, quantities, prices and terms match your quote, and raise any differences before you start work.
What is a blanket purchase order?
A blanket purchase order covers repeated purchases over a set period, often with a maximum value, instead of a new PO for every order. You quote the same PO number on each invoice raised against it. Track how much of the blanket PO you have already billed, because invoices that would take the total over its limit or past its end date are often rejected.
Can I start work before I receive a PO number?
You can, but it is risky with organizations that use purchase orders. Without a PO, the purchase may not be approved in their system, and your invoice may be held until someone raises one. If the work is urgent, get written confirmation from an authorized person that a PO is coming, and ask for the number before you invoice.
What is three-way matching?
Three-way matching is a check accounts payable runs before paying: it compares the purchase order, the goods receipt or delivery record, and your invoice. Quantities, prices and items should agree across all three. If you invoice for more than was received or at a different price, the invoice is held. Two-way matching compares only the PO and the invoice.
What happens if I put the wrong PO number on an invoice?
The invoice will usually fail matching and be rejected or put on hold. Contact accounts payable to ask how they want it fixed. Many will ask you to cancel the original with a credit note and issue a new invoice with the correct PO number. Don't edit and resend the same invoice number with different details, as that confuses both sets of records.
Do government clients always require a PO number?
Public sector bodies very often use purchase orders and may refuse to pay without one, but requirements vary by country and organization. Many government buyers also require invoices to be submitted through a specific e-invoicing or procurement portal. Ask during onboarding what they need on the invoice and where to send it, and follow their instructions exactly.
What is the difference between a supplier number and a PO number?
A supplier or vendor number identifies you in the buyer's system and stays the same across all your orders. A PO number identifies one specific approved purchase. Some buyers ask you to show both on every invoice. Your supplier number is usually assigned when you complete their new supplier setup, so ask for it if you haven't received one.
Can I split one PO across several invoices?
Yes, partial invoicing against one PO is common for phased projects or staggered deliveries. Quote the same PO number on each invoice and describe which part of the order it covers, such as Phase 2 of 3. Make sure the combined total of your invoices never exceeds the PO value, and keep a running record of what has been billed.
Invoice Creator Editorial Team
Guides on invoicing, cash flow and small-business finance from Invoice Creator, operated by Next Software Development Company. Content is reviewed for accuracy against the fields our own invoice generator produces.
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