Purchase Order vs Invoice: What Is the Difference and When Do You Need Each?

By The Invoice Makers team · Updated November 1, 2026 · 8 min read

A purchase order (PO) is a document the buyer sends to a seller to order goods or services at an agreed price. An invoice is a document the seller sends to the buyer after delivery to request payment. In short, the PO comes first and says "this is what we want to buy," and the invoice comes later and says "this is what you owe." When both carry the same PO number, the buyer's accounts team can match them and pay quickly.

Key takeaways

• The buyer creates the purchase order; the seller creates the invoice.
• A PO is issued before the work or delivery; an invoice is issued after it.
• Once the seller accepts a PO, it usually becomes a binding agreement on price, quantity and terms.
• Many companies use three-way matching (PO, receiving record, invoice) before they approve payment.
• If a client gives you a PO number, put it on your invoice. Missing PO numbers are one of the most common reasons invoices get stuck.

What is a purchase order?

A purchase order is a formal request from a buyer to a supplier. It lists exactly what the buyer wants, how many, at what price, and when and where it should be delivered. Companies use POs to control spending: someone inside the business approves the purchase before any money is committed, and finance can see what has been ordered but not yet billed.

A typical PO includes:

• A unique PO number, such as PO-2026-0148
• The buyer's name, billing address and delivery address
• The supplier's name and contact details
• Item descriptions, quantities, unit prices and line totals
• Delivery date and shipping terms
• Payment terms, for example Net 30
• The name or signature of the person who approved it

If you buy supplies from vendors yourself, you can create one in a couple of minutes with the purchase order generator.

What is an invoice?

An invoice is the seller's bill. It confirms what was delivered and asks for payment by a specific date. It is also an accounting and tax record for both sides: the seller records it as income owed, and the buyer records it as an expense to pay.

A good invoice shows your business details, the client's details, a unique invoice number, the invoice date, a due date, itemized charges, any tax, the total and how to pay. If the client issued a PO, the invoice should also reference that PO number. For a full field-by-field walkthrough, see how to write an invoice.

Purchase order vs invoice: the differences at a glance

The easiest way to remember it: a PO is about permission to buy, and an invoice is about permission to be paid.

How the PO-to-payment process works

In most businesses, a sale that starts with a purchase order follows the same five steps. Seeing the full chain helps you understand why your invoice might be waiting in someone's approval queue.

1. Quote. The seller sends a price for the work or goods. The quote generator is handy here. (Some buyers skip this step for repeat orders.)
2. Purchase order. The buyer issues a PO based on the quote. The seller checks it and accepts it, in writing or simply by fulfilling it.
3. Delivery. The seller ships the goods or completes the work. For physical goods, a packing slip goes in the box so the buyer can check what arrived. The buyer records what was received.
4. Invoice. The seller sends an invoice that references the PO number and bills for what was actually delivered.
5. Payment. The buyer matches the documents, approves the invoice and pays by the due date. The seller can confirm with a receipt; our guide to invoice vs receipt explains the difference.

What is three-way matching?

Three-way matching is a check that accounts payable teams run before paying a supplier. They compare three documents line by line:

• The purchase order: what was ordered and at what price.
• The receiving record (goods received note or delivery confirmation): what actually arrived.
• The invoice: what the supplier is charging.

If quantities and prices agree across all three, the invoice is approved. If they do not, it is flagged and usually held until someone sorts out the difference. Some companies only use two-way matching (PO and invoice) for services, where there is no physical delivery to count.

Example: a three-way match that fails

Riverside Cafe issues PO-2026-0148 to a supplier for 200 branded coffee cups at $1.20 each and 10 aprons at $18.00 each, a total of $420.00. Only 180 cups arrive; the other 20 are back-ordered.

The invoice bills for 200 cups, but the receiving record shows 180, so the whole invoice goes on hold. The fix is to invoice for what shipped: 180 cups at $1.20 is $216.00, plus $180.00 for aprons, for a total of $396.00. The remaining 20 cups ($24.00) go on a second invoice once they ship. If you already sent the $420.00 invoice, issue a credit note for $24.00 rather than editing the original.

Why the PO number belongs on your invoice

For a supplier, the PO number is the key that unlocks payment. Large organizations often receive thousands of invoices a month, and their systems match each one to an open PO automatically. An invoice without a PO number may be rejected outright, returned to you, or left sitting until someone works out what it relates to. That can add weeks to a Net 30 term.

Good practice:

• Put the PO number in a clearly labeled field near the top: "PO number: PO-2026-0148."
• Copy the item descriptions and unit prices exactly as they appear on the PO.
• Send one invoice per PO unless the client tells you otherwise.
• Keep your own invoice number separate. The PO number is the client's reference; your invoice number is yours. See how to number invoices for a simple system.
• Ask for the PO number before you start work. It is much easier to get it at the beginning than when the invoice is already overdue.

If a client still pays late, our guide on how to ask for payment includes reminder templates, and the first thing to check is whether the PO number was on the invoice.

Does a small business need purchase orders?

Not always. A freelancer or a three-person shop that buys a few things a month can usually manage with quotes, receipts and a spreadsheet. But POs start to pay for themselves when:

• More than one person can buy things. A PO with an approval step stops duplicate orders and surprise bills.
• You order stock regularly. POs give you a record of what is on order, what has arrived and what is still outstanding.
• You work with suppliers on credit terms. A written PO settles arguments about agreed prices and quantities.
• You need to track budgets. Open POs show money you have committed but not yet paid.
• Your bank, investor or grant funder asks for spending controls. POs are a simple, auditable way to show them.

On the selling side, you do not need to issue POs at all. But if you sell to larger companies, schools, hospitals or government bodies, expect them to send you one and to insist that your invoice matches it.

Common mistakes to avoid

• Starting work without a PO when the client's policy requires one. Some organizations will not pay invoices for work done before the PO date.
• Invoicing more than the PO value. If the scope grows, ask the client to issue a revised PO or change order first.
• Changing prices on the invoice. Any price that differs from the PO will fail the match, even if it is lower.
• Treating a PO as an invoice. A PO is not a request for payment. The buyer will not pay until you send an invoice.
• Forgetting tax. Some POs show prices before sales tax. Check whether tax should be added; the sales tax calculator helps with the math.

Create both documents in minutes

A purchase order and an invoice are two halves of the same transaction: one authorizes the purchase, the other collects the money. When they agree, payment flows smoothly. Use the purchase order generator when you are buying, and the free invoice generator when you are selling. Add the PO number, set a due date (the payment due date calculator works it out from your terms), choose a design and download a PDF ready to send.