The Document That Controls Your Spending
Most businesses obsess over the invoices they send out, because that is where the money comes from. Far fewer pay the same attention to the money going the other way. That is a mistake, because the document that controls spending sits right at the start of every purchase, and getting it right is the difference between a supplier bill you expected and one that quietly overcharges you.
That document is the purchase order. It is one of the most common pieces of paperwork in business-to-business trade, yet plenty of owners have never issued one, and plenty more confuse it with the invoice that follows. This guide explains what a purchase order is, what it should contain, the four main types, how a PO differs from an invoice and a requisition, and how it flows into your bookkeeping. Plain English throughout.
What Is a Purchase Order?
A purchase order, usually shortened to PO, is a document a buyer sends to a supplier to formally request goods or services at an agreed price. It sets out exactly what the buyer wants, how much of it, at what price, and when and where it should be delivered. In short, it is the buyer putting their order in writing before anything changes hands.
The important part is what happens next. A purchase order on its own is just an offer to buy. Once the supplier accepts it, the PO becomes a legally binding contract between the two parties, on the terms written into the order. That is why a PO matters more than an email or a phone call: it fixes the price and the quantity in advance, so neither side can move the goalposts later.
Think of it as the mirror image of an invoice. The invoice is the seller asking to be paid after the work is done. The purchase order is the buyer confirming what they will pay for before the work starts. One opens the transaction, the other closes it.
Purchase Order Meaning: Why Bother With One?
If you already agree prices by email, a formal PO can feel like extra admin. In practice it earns its place for four reasons.
It creates a clear record. When a delivery arrives or a bill lands, you have a dated document stating what you actually ordered. Disputes about quantity, price or spec get settled in seconds rather than arguments.
It controls spending before it happens. A PO has to be raised, and usually approved, before an order goes out. That approval step is where overspending gets caught, because someone has to sign off the commitment while it is still just a request, not a debt.
It gives your accounts a matching reference. Every PO carries a unique number, and that number ties the order, the delivery and the supplier invoice together. We come back to why that matters below.
It protects you legally. Because an accepted PO is a binding contract, a supplier who delivers the wrong thing or invoices a higher price is in breach of agreed terms, and you have the paperwork to prove it.
What a Purchase Order Should Include
A purchase order is mostly a checklist, and a good one captures the same fields every time. At a minimum a PO should show:
- A clear label that the document is a purchase order.
- A unique purchase order number.
- The date the order is raised.
- Your business name, address and contact details as the buyer.
- The supplier's name and address.
- A description of each item or service, with quantities and agreed unit prices.
- The total value of the order, and how VAT is being treated.
- The delivery address and the date or deadline for delivery.
- The payment terms, such as 30 days from invoice.
None of this needs dedicated software. Most accounting packages, including Xero, can raise and track purchase orders as standard, but a consistent template does the same job for a smaller business. What matters is that the same details are captured every time, because that consistency is what makes matching the invoice later a quick job rather than a hunt.
What Is a Purchase Order Number?
The purchase order number is the unique reference given to each PO, and it does more work than its size suggests. It is the thread that runs through the whole purchase.
When the supplier accepts your order, they quote your PO number back to you. When the goods arrive, the delivery note carries it. When the invoice comes in, the supplier prints the same PO number on it, often in a field labelled as the customer or buyer reference. That single number lets your accounts team line up three separate documents and confirm they all describe the same transaction.
This is also why a PO number on an invoice is not decoration. If a supplier invoices you without the PO number you gave them, your accounts payable process cannot match it automatically, and the bill sits in a query pile until someone works out which order it belongs to. Insisting that suppliers quote your PO number is one of the simplest controls a growing business can put in place.
Types of Purchase Order
Not every purchase looks the same, so there are a few recognised types of purchase order. Four come up most often.
A standard purchase order is the everyday version. You know exactly what you want, how much, and when, so you raise a one-off order for that specific purchase. Most POs a small business issues are standard ones.
A planned purchase order is used when you know what you will need and roughly the total quantity, but not the exact delivery dates. You agree the items and prices up front, then release deliveries against the order as you need them.
A blanket purchase order, sometimes called a standing order, sets up an ongoing arrangement with a supplier over a period, often at agreed prices, without committing to fixed quantities in advance. It suits recurring supplies where you draw down as required and reconcile at the end of the period. The trade-off is that a blanket PO needs watching, because open-ended commitments are easy to lose track of.
A contract purchase order is a longer-term agreement that fixes the terms of the relationship, such as pricing and conditions, which individual future orders then reference. It sits behind the day-to-day standard POs rather than ordering anything itself.
For most owner-managed businesses, standard POs cover almost everything, and blanket orders come in once you have a regular supplier you buy from repeatedly.
Purchase Order vs Invoice
This is the confusion we see most, so it is worth being precise. A purchase order and an invoice sit at opposite ends of the same transaction.
| Purchase order | Invoice | |
|---|---|---|
| Who raises it | The buyer | The seller |
| When | Before the goods or services are supplied | After they are supplied |
| Purpose | Requests and authorises the purchase | Requests payment for it |
| Effect in the buyer's books | A commitment, not yet a cost | Records the cost and the amount owed |
The buyer creates the PO to say what they want. The seller creates the invoice to ask for payment once the order is fulfilled. They are not alternatives, and a well-run purchase uses both: the PO opens it, the invoice closes it, and they share a number so anyone can see they belong together.
A purchase order is also not the same as a purchase requisition. A requisition is an internal request from a member of staff to their own finance or purchasing team asking for permission to buy something. If that request is approved, a purchase order is then raised and sent out to the supplier. The requisition stays inside the business, the PO goes out to the supplier.
The Purchase Order Process, Start to Finish
Put together, a purchase order runs through a predictable cycle. Understanding the full flow is what turns a PO from a piece of paper into a genuine financial control.
- Requisition. Someone in the business identifies a need and requests the purchase internally.
- Purchase order raised. Once approved, a PO is created with a unique number and sent to the supplier.
- Supplier accepts. The supplier confirms the order, at which point it becomes a binding agreement.
- Goods or services delivered. The supplier fulfils the order and issues a delivery note or goods received note, quoting the PO number.
- Invoice received. The supplier sends an invoice, again referencing the PO number.
- Three-way match. Accounts payable check the purchase order, the delivery note and the invoice against each other.
- Payment. Once the three documents agree, the invoice is approved and paid on the agreed terms.
Step six is the one accountants care about most. Three-way matching means the invoice is only paid if it agrees with what was ordered (the PO) and what was actually received (the delivery note). If a supplier invoices for ten units but the PO and delivery note both say eight, the mismatch is caught before any money leaves the business. It is one of the most effective everyday controls against overcharging, duplicate billing and simple error, and it only works if you raised a PO in the first place.
Purchase Orders in Accounting
Here is the point most generic guides skip, and where the accounting treatment surprises people. Raising a purchase order does not create an entry in your accounts.
That catches business owners out, because an invoice does the opposite. When you receive a supplier invoice, that cost is recorded straight away and the amount sits in trade payables until you pay it, following the rules of double-entry bookkeeping. A purchase order records nothing, because at the point you raise it, nothing has actually happened yet. No goods have arrived, no service has been delivered, and no liability exists. Under accrual accounting, a cost is only recognised when the goods or services are received, not when they are ordered.
So a PO is what accountants call a commitment rather than a transaction. The bookkeeping entries begin the moment the goods are received or the invoice arrives, not when the order goes out. That said, an open PO is still valuable financial information, because it tells you what you are committed to spend even though it has not hit the accounts yet. Larger organisations track this formally as commitment accounting, so their budgets reflect orders placed as well as bills received. A small business rarely needs that level of formality, but the principle is worth borrowing: your bank balance does not show the orders you have already agreed to pay for, and forgetting them is how cash flow surprises happen.
One more practical point on VAT. The VAT on a purchase is reclaimed from the invoice, not the purchase order, so the VAT figure on a PO is an estimate for your own planning. You only recover input VAT once you hold a valid VAT invoice, which is another reason the invoice, not the order, is the document your accounts are built on.
Do Small Businesses Actually Need Purchase Orders?
In our experience the honest answer is: not always at first, but sooner than most owners think. A sole trader buying occasional supplies does not need a formal PO system, and forcing one in would be pointless admin. The picture changes the moment you have more than one person able to spend company money, or you buy from the same suppliers regularly, or a single order gets large enough that a pricing mistake would hurt.
That is the tipping point where purchase orders stop being bureaucracy and start being a control. They put an approval step in front of spending, they give you leverage when a supplier bills the wrong amount, and they make your purchase ledger something that reconciles cleanly rather than a pile of bills you half-remember agreeing to. If your supplier invoices have started arriving faster than you can check them, that is usually the sign it is time.
How IAK Can Help
Purchase orders are simple in theory and easy to let slide in practice. The value is not in the document itself but in the discipline around it: raising them consistently, insisting suppliers quote your PO number, and matching every invoice back to what you ordered and received before you pay it.
That is exactly the kind of routine our bookkeeping and accounting teams build for clients, usually inside Xero so the whole purchase-to-payment trail lives in one place. We set up your purchase ledger so orders, deliveries and invoices tie out every month, flag the mismatches before they cost you, and keep your VAT records clean for reclaim. If you are not sure whether your business has outgrown paying bills on trust, that is a good conversation to have. Learn more about what an accountant actually does day to day, and get in touch when you are ready.
Sources
- Purchase order, Wikipedia, on the definition, legal status and formats of purchase orders.
- Charge, reclaim and record VAT: VAT invoices, GOV.UK, on why input VAT is reclaimed from the invoice rather than the order.
- VAT record keeping, GOV.UK, on the purchase records a VAT-registered business must keep.