What is a PO number?
A PO number (purchase order number) is the unique reference a buyer's system gives to each purchase order. The vendor quotes it on the delivery challan and the invoice so the buyer can match the bill to what was ordered and received.
On this page
PO stands for purchase order, so a PO number is simply the number printed on a purchase order. It identifies one order from one buyer to one vendor. If you sell to companies you will be asked for it constantly, usually in an email from their accounts team that says they can't process your invoice without it.
What a PO number looks like
There is no standard format. Every ERP or procurement tool picks its own. A few patterns you will see in India:
- PO-2026-00481: a prefix, the year and a running number.
- PO/25-26/0481: the financial year instead of the calendar year, common because Indian books close on 31 March.
- PUN-PO-0481: a plant or branch code in front, used by companies that buy separately for each location.
- 4500012345: a plain ten-digit number. SAP systems often start PO numbers with 45.
The format matters less than the rule behind it. Each number is used once, and it doesn't change after the PO is issued, even if the order is amended. Amendments usually get a revision suffix (PO-2026-00481 Rev 1) so both sides know which version they are looking at.
Who creates the PO number
The buyer does. More precisely, the buyer's software does, at the moment a purchase order is approved and released. A vendor never makes up a PO number. If a customer asks you to 'raise a PO', they have the roles mixed up. What they need from you is a quotation or a proforma invoice, and their purchasing team then issues the PO.
Where the PO number shows up
Once issued, the same number travels with the order through every document:
| Document | Prepared by | Why the PO number is on it |
|---|---|---|
| Purchase order | Buyer | This is where the number starts. |
| Order acknowledgement | Vendor | Confirms which order the vendor has accepted. |
| Delivery challan | Vendor | Lets the stores team find the order when the truck arrives. |
| Goods receipt note (GRN) | Buyer | Records what was received against that PO. |
| Tax invoice | Vendor | Lets accounts payable match the bill to the PO and GRN. |
| Payment advice | Buyer | Tells the vendor which invoices and orders a payment covers. |
That chain is what makes three-way matching possible. Accounts compares the PO, the GRN and the invoice line by line. If the invoice has no PO number, someone has to hunt for the right order by vendor name and amount, and that is how invoices end up parked for weeks.
PO number, PR number and invoice number
These get mixed up all the time, mostly because they appear on the same screens.
- The PR number is the internal reference for a purchase requisition, the request raised before any order exists. Vendors normally never see it.
- The PO number is the buyer's reference for the order sent to the vendor.
- The invoice number is the vendor's own reference for their bill. Under GST it has to be unique within the financial year and can be at most 16 characters long.
If you are the vendor
- Ask for the PO before you dispatch, not after. Once the goods have left, you have less to bargain with and the buyer's team is in less of a hurry.
- Give the PO number its own field on the invoice instead of burying it in the item description.
- Stick to one invoice per PO where you can. Invoices that cover three POs, or half of one, are the ones that get stuck.
- If the buyer changes the quantity or price, ask for a revised PO. Billing a price that isn't on the PO is the most common reason invoices get rejected.
If you are the buyer
Tell vendors in writing that invoices without a valid PO number will be returned. It feels strict, but it's the simplest control you have against maverick spend, and once vendors know the rule they follow it. Keep the exceptions (utilities, rent, statutory payments) on a short written list instead of deciding case by case.
Frequently asked questions
What does PO number mean on an invoice?
It is the number of the purchase order the invoice is billing against. The buyer uses it to match the invoice to the order and the goods receipt before paying.
Is a PO number the same as an invoice number?
No. The buyer issues the PO number when they place the order. The vendor issues the invoice number when they bill. One PO can be billed across several invoices if it is delivered in parts.
Who provides the PO number, the buyer or the seller?
The buyer. It comes from the buyer's purchasing system when the purchase order is approved. Sellers should ask for it before they ship.
What happens if an invoice has no PO number?
In most companies it is sent back or held until someone finds the matching order. Some firms accept invoices without a PO for a short list of categories like rent or utilities, but for goods and services bought through purchasing the PO number is usually mandatory.