What Is a Purchase Order and When Do You Actually Need One?

A client emails you after agreeing on the project scope and says, "We just need a PO number before we can process anything." You stare at the message for a moment, unsure whether to ask what that means or pretend you already know. If you have ever been in that position, you are not alone. Purchase orders come up constantly in freelance work, and most people learn about them only when a client asks for one.

What a purchase order actually is

A purchase order is a document that a buyer sends to a supplier to formally request goods or services. It is not a payment. It is not a promise to pay. It is an official record that a purchase is being made, at what price, and under what terms. Once you as the seller accept it, the PO becomes a binding commitment from the buyer to pay you if you deliver what is described.

Think of it as the client's version of a contract. Where you might send a quote to say "here is what I will do and for how much," the client sends a purchase order back to say "yes, proceed, on these terms, under this reference number."

Large companies use purchase orders because they have finance departments that need to track every outgoing payment. The PO gets logged in their accounting system before any money leaves. When your invoice arrives later, someone in accounts payable checks that your invoice references a valid PO before releasing the payment. No PO reference on your invoice often means your invoice sits in a queue going nowhere.

How a purchase order differs from an invoice

These two documents sit at opposite ends of the same transaction. A purchase order comes first, from the buyer, before the work happens. An invoice comes from you, the seller, after the work is done.

The PO says: we want this, at this price, delivered by this date. Your invoice says: we did this, the price is this, please pay by this date.

They should match each other. The line items, quantities, and amounts on your invoice should correspond to what was authorized in the PO. Many corporate clients will reject an invoice that does not reference the PO number or that bills for more than was approved.

When clients ask for one

Not every client will ask for a purchase order. Individuals and very small businesses rarely use them. Sole traders booking a photographer or a copywriter usually just agree a fee by email and then pay the invoice. No PO involved.

The clients who use purchase orders tend to be companies with formal procurement systems: marketing agencies, corporate businesses, public sector organizations, universities, and large nonprofits. If you do any work for a company with more than 50 employees, or if your contact is in a marketing or operations department rather than being the owner, there is a good chance they have a PO process.

You will often know a PO is required because your client will tell you before work starts. Sometimes they ask for it mid-project, which can create delays. If you are pitching to a larger company, it is worth asking during the negotiation phase whether they need a PO raised before you can begin.

What to do when you receive a PO

If a client sends you a purchase order, read it carefully. Check that the scope description matches what you agreed. Check the amount, the currency, and any payment terms listed. If anything looks different from what you discussed, raise it before you start work, not after.

Once you accept the PO, make note of the PO number. You will need to put that number on every invoice you send for that project. Put it prominently, either in a dedicated field on your invoice or in the notes section. Something like "PO Reference: PO-2026-4481" placed near the top of the invoice will satisfy most accounts payable departments.

If the PO authorizes a certain total and the project later expands, do not invoice for the higher amount without getting an updated PO first. Going over the authorized amount without a new PO is one of the most common causes of delayed payment on corporate projects.

What if you need to raise a purchase order yourself

Some freelancers who run their own small operations end up needing to issue purchase orders too, usually when they are subcontracting work or ordering supplies. If a photographer needs to buy specialist equipment for a shoot and bill the cost through to the client, a PO from the client helps justify the expense.

If you need to create a purchase order, the purchase order generator on Invoice No. gives you a clean PDF you can issue to your own suppliers. It covers all the standard fields: buyer and supplier details, line items, delivery information, and a reference number.

The practical short version

A purchase order is the client's written commitment to pay you for something specific, before you do the work. When a client sends you one, note the PO number and put it on your invoice. When a client asks you to raise one before they can pay, they are asking you to formally request the purchase through their system. Either way, the document exists to create a paper trail that protects both sides.

If you work regularly with companies rather than individuals, understanding POs will save you a lot of confused back-and-forth with accounts payable.

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