A purchase order (PO) is the document a buyer sends a supplier to order goods, what you want, how many, at what price, delivered where and when. Once the supplier accepts it, it is a binding order. An invoice comes later; the purchase order comes first.
Purchase order meaning, in plain English
A purchase order is your formal request to buy. Instead of phoning the supplier and hoping the right thing turns up, you put it in writing: the product, the quantity, the price you agreed, where to deliver and when you need it. The supplier accepts it, amends it or declines it. Once accepted, a PO is a contract, not a wish list.
The order of events is what trips most people up. A purchase order is raised by the buyer before anything ships. An invoice is raised by the supplier once the goods are on their way. The delivery note travels with the goods. Checking all three against each other, PO, delivery note, invoice, is called a three-way match, and it is the control that stops you paying for something you never received.
What goes on a purchase order
- A PO number, a unique reference, so the order and its matching invoice can be found again months later.
- Supplier details, the legal entity you are ordering from, not just the account manager whose name is on the email.
- Line items, product or SKU, description, quantity, unit of measure and unit price, one row per line.
- Delivery and payment terms, the delivery address, the date you need the goods, and the terms you agreed (30 days, pro-forma, and so on).
- Totals, line totals, order value, VAT and any delivery charge.
The line that decides everything: order quantity
When people argue about a purchase order, they usually argue about price. Price is not the expensive number. Quantity is. Order too little and the line stocks out before it replenishes; order too much and the surplus sits on a shelf as working capital you cannot spend anywhere else. Multiply that one decision across thousands of products and hundreds of orders a year and it dwarfs any unit-price negotiation.
A worked example
Say a wholesaler is replenishing a steady line. Average demand is 40 units a day, the supplier takes 21 days to deliver, and the buffer has to cover the normal wobble in demand:
- Demand over the lead time: 40 × 21 = 840 units
- Safety stock at a 95% service level: ≈ 113 units (the safety stock calculator shows the working)
- Stock already on hand: 300 units; already on order: 200 units
- Order quantity ≈ 840 + 113 − 300 − 200 = 453 units, then rounded up to a valid multiple of the supplier minimum
That last step is the one manual buying usually gets wrong. If the supplier sells in cartons of 24, 453 becomes 480, a number you can actually place, not a number that ignores the supplier terms.
Purchase order vs invoice vs sales order
Three documents, three directions of travel. Getting them the wrong way round is the single most common confusion on a PO:
| Purchase order | Invoice | Sales order | |
|---|---|---|---|
| Raised by | Buyer (you) | Supplier | Buyer (you), sent to your customer |
| Timing | Before the goods ship | After the goods ship | Before the goods ship |
| What it does | Instructs a supplier to send goods at an agreed price | Requests payment for goods already sent | Confirms an order a customer placed with you |
| Money direction | You owe it | You pay it | You are owed it |
Purchase order numbers, explained
A PO number is a reference that lets you tie an order to its confirmation, its delivery note and its invoice without ever opening a drawer. There is no single legal format, but the conventions that age well are simple: a short prefix, the year, and a sequential number, PO-2026-0417. Never reuse a number, and never reuse a cancelled one either, because a reused reference is how two orders quietly become one in your accounts.
Why purchase orders matter for stock control
An open PO is stock you own but cannot sell yet. If your system does not subtract what is already on order when it suggests a reorder, you will double-order, and find out when two pallets arrive instead of one. That single flaw, on a line that runs every few weeks, is often the whole reason overstock builds up in the first place.
Optimal Chain decides what goes on the purchase order before the paperwork starts. Give it your budget, say £250,000 for the quarter, and the Replenishment Budget Planner allocates it across every product and supplier to the penny, funding the most urgent revenue first and respecting every supplier minimum. Approved orders export as a formatted CSV or Excel sheet ready to import into your ERP, so the plan you agreed is the plan you place.