Purchase Order vs Invoice: What's the Difference?

A purchase order and an invoice are two halves of the same business transaction, which is exactly why they get confused. One starts the deal, the other ends it. One is created by the buyer, the other by the seller. Mixing them up, or worse, not using them where a client expects them, can slow your payment right down, because many businesses will not pay an invoice that does not reference their purchase order. Understanding how the two fit together is essential once you sell to companies rather than individuals.

This guide explains purchase order vs invoice: who creates each, when, what they are for, how they work together, and why the PO number matters. It works for freelancers and small businesses selling to other businesses.

Quick comparison

	Purchase Order (PO)	Invoice
Created by	The buyer	The seller
When	Before the goods/work	After the goods/work
Purpose	Authorises and orders a purchase	Requests payment
Contains	What is ordered, agreed price	What is owed, tax, due date
Legally	An offer to buy	A demand for payment

In one line: the buyer sends a purchase order to order and approve the purchase, and the seller sends an invoice to get paid for it. The PO comes first, the invoice comes after.

What is a purchase order?

A purchase order is a document the buyer creates to formally order goods or services from a supplier. It lists what they want, the quantities, the agreed price, and a unique PO number. When the supplier accepts it, the PO becomes a binding agreement to supply at those terms.

Purchase orders matter to businesses because they control spending: someone with authority approves the PO before money is committed, and the finance team uses it to check that later invoices match what was actually ordered. For a small supplier, receiving a PO is good news, it means the purchase is approved and budgeted.

What is an invoice?

An invoice is the document the seller creates after delivering the goods or work, to request payment. It carries a unique invoice number, the amount owed, tax if registered, payment terms, and, crucially in a B2B deal, a reference to the buyer's PO number. The invoice is the formal record of the debt and the tax point.

Where the PO says "we would like to buy this," the invoice says "here is what you now owe, please pay by this date." For everything an invoice must contain, see our invoice format and layout guide and our complete guide on how to make an invoice.

How a PO and invoice work together

The two documents are stages of one process:

1. The buyer issues a purchase order with a PO number, approving the purchase.
2. The supplier delivers the goods or completes the work.
3. The supplier sends an invoice that references the PO number, so finance can match them.

This matching is why the PO number is so important. Many accounts departments run a "three-way match", checking the purchase order, the delivery record, and the invoice all agree before releasing payment. An invoice that does not quote the PO number often gets held up or bounced back, delaying your money.

Why the PO number matters on your invoice

If a business client gives you a purchase order, always put the PO number on your invoice. It is the single most common reason B2B invoices get paid on time versus stuck in a queue:

• It lets finance match your invoice to the approved order instantly.
• It proves the purchase was authorised, so no one has to chase an approval.
• It routes your invoice to the right budget and person.

Leaving the PO number off, when the client uses POs, is a self-inflicted payment delay. Ask for the PO number upfront if a business client has not given you one. Missing references are a frequent entry in our common invoice mistakes guide.

What to do when you receive a purchase order

Getting a PO from a client is a good sign, but handle it properly:

• Check the details match what you agreed. The items, quantities, and prices on the PO should line up with your quote. If anything is wrong, query it before you start, not after, since your invoice will have to match the PO to get paid.
• Record the PO number somewhere you will not lose it, because it must appear on your invoice.
• Only supply what the PO covers. If the client wants more, ask them to raise a new or amended PO rather than adding work informally.

Treating the purchase order as the controlling document, and matching your invoice to it exactly, is what keeps a business client's finance team happy and your payment on schedule.

When do you need a purchase order?

POs are mostly a B2B and larger-organisation thing. You will encounter them when:

• Selling to medium or large companies, government, or institutions, which usually require a PO.
• Fulfilling repeat or bulk orders, where the PO controls each order.
• Working on approved budgets, where spending must be authorised first.

Selling to individuals or very small businesses, you often will not see a PO at all, the quote and invoice are enough. As you take on bigger clients, POs become normal, and handling them smoothly signals you are used to working with businesses. For clients abroad, PO and invoice conventions vary, see our how to invoice international clients guide; tax rules by country are on our UK, Australia, and USA pages.

Common confusions

• Thinking the PO is the invoice (or vice versa), when they have opposite jobs.
• Leaving the PO number off the invoice, delaying payment.
• Sending an invoice with no PO to a client who requires one.
• Prices not matching between PO and invoice, which stalls the three-way match.
• Treating a PO as a payment, when it only authorises the purchase.

Invoice against a PO cleanly

When a client gives you a purchase order, your invoice just needs to match it and quote the PO number. Invoicara's free invoice generator lets you add a PO reference, itemise exactly what was ordered, handle tax and totals, and export a clean, matchable PDF that sails through a finance team's checks. No sign-up, no watermark, free forever.

For related distinctions, see our invoice vs receipt vs bill guide and invoice vs quote vs estimate guide. Remember the simple split: the buyer's purchase order orders and approves, and the seller's invoice requests payment, with the PO number tying them together.