Put a purchase order and an invoice side by side and you would struggle to tell them apart. Same layout, same line items, often the same numbers. No wonder people use the words interchangeably. But they are created by different people, at different moments, for opposite reasons, and understanding that difference is what keeps your accounts clean and your suppliers honest. Here is the whole thing in plain terms.
The purchase order: the buyer's promise
A purchase order, or PO, is sent by the buyer to the seller to confirm an order before anything is delivered. In effect it says, "I want these items, in these quantities, at these prices." Once the seller accepts it, the PO becomes a commitment and gives both sides a shared reference for the order. It is the document that turns "can you send me a hundred units?" into something official that finance can approve and track.
The invoice: the seller's request for payment
An invoice runs the other way. The seller sends it to the buyer after the goods or services have been delivered, and it says, "here is what I supplied, and here is what you owe." Unlike a PO, an invoice carries payment terms and a due date, and it becomes part of both parties' accounting records. In short, the PO orders and the invoice collects.
The differences that matter
- Who creates it: the buyer writes the PO, the seller writes the invoice.
- When: the PO comes first, before delivery; the invoice comes after.
- Purpose: a PO confirms an order, an invoice requests payment.
- Legal weight: an accepted PO commits the buyer to buy, an invoice creates an obligation to pay.
- Contents: both list items and prices, but only the invoice carries payment terms and a due date.
How they work together
In a tidy transaction the two documents form a clean trail. The buyer issues a PO with a unique number. The seller accepts it and delivers. The seller then invoices, quoting that same PO number. The buyer checks the invoice against the PO and the delivery before paying anything. That check has a name, three-way matching, and it is one of the simplest fraud controls a business has. Carrying the PO number onto the invoice is what makes it possible, so never skip it.
Why the distinction is worth caring about
Get this right and your books stay accurate and your suppliers stay happy. Purchase orders give you control over spending and a record of what you have committed to. Invoices give you the legal footing to collect and the data your bookkeeper needs. Businesses that skip POs tend to lose track of what they have agreed to buy, while businesses that drag their feet on invoices create their own cash-flow problems. Neither document is glamorous, but together they are the backbone of getting paid and staying solvent.
What each one should contain
A complete purchase order has a unique PO number, buyer and seller details, an itemised list with agreed prices, the delivery address and date, and any agreed terms. A complete invoice has a unique invoice number, buyer and seller details, the referenced PO number, an itemised list, the tax breakdown, the total due, the payment terms and the due date. Keep the line items consistent between the two, flag any agreed change, and your paperwork becomes an audit-ready record instead of a source of confusion.
When a small business should start using POs
Plenty of one-person businesses get by without purchase orders, and that is fine at first. The moment to introduce them is when more than one person can spend the company's money, or when you start losing track of what you have committed to buy. A PO adds a simple approval step before cash goes out the door, which protects your budget and makes your bookkeeper's life much easier. You do not need an expensive procurement system to start, either. A numbered purchase order sent before each significant order is enough to bring order to your spending and give you a clean record to match invoices against later.
Frequently asked questions
Can I send an invoice without a purchase order?
Yes. Plenty of small transactions skip the PO and go straight to an invoice. POs are most common in larger organisations that need formal spending approval.
Is a purchase order a legal contract?
Once the seller accepts it, a PO generally becomes a binding agreement to buy and sell on the stated terms.
What is a PO number for?
It is a unique reference that ties the order, the delivery and the invoice together, so anyone can match the documents and resolve a query quickly.
Which comes first, the PO or the invoice?
The purchase order, always. It orders the goods; the invoice, which follows delivery, requests payment for them.
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