What Is a PO Number on an Invoice? Where It Goes and Why

A PO number on an invoice is the buyer's purchase order number, copied onto the supplier's invoice so the buyer's accounts team can match the bill to what they ordered and pay it. It belongs to the client, not to you: you don't make it up, you copy it exactly from the purchase order they sent. Leave it off, or get it wrong, and many company clients will send the invoice back unpaid.

Where the PO number goes on an invoice

Put the PO number in the invoice header, beside your own invoice number and date, with a clear label. That is where an accounts clerk looks first, and many companies scan invoices automatically and read it from there. If the job was covered by more than one purchase order, list each PO number against the lines it covers.

In our invoice generator, type "PO number: PO-58821" and the PO date into the notes, or into the first line item description, so they print on the PDF you send. Copy the number character for character, including any letters, dashes or leading zeros. "PO-58821" and "58821" can look like two different orders to a matching system.

What the PO date means

The PO date is the date the buyer issued the purchase order. It is printed on the PO they sent you. Some clients ask for it on the invoice as well, because they file purchase orders by date or have several POs with similar numbers. It is not your invoice date, and it doesn't change your payment terms, which still run from the invoice date unless your agreement says otherwise.

How to invoice against a purchase order

Invoicing against a purchase order means your invoice mirrors what the PO approved. The buyer's accounts team will usually check three things: the purchase order, proof the goods or work were delivered, and your invoice. If all three agree, you get paid. Many suppliers also confirm the order before delivery with a sales order that quotes the PO number, which our sales order generator makes in a few minutes.

1. Copy the PO number and date into the invoice header.
2. Use the same line descriptions as the purchase order, so each line can be ticked off against it.
3. Match the quantities and unit prices. If you delivered less, invoice for what you delivered and note the balance still to come.
4. Don't add unapproved extras. Extra work needs a new or amended PO first, or the whole invoice may be held.
5. Send it to the address on the PO, which is often an accounts payable inbox, not your contact person.

What happens if the PO number is missing or wrong

At a company with a purchasing process, an invoice without a valid PO number usually can't be matched to an approved order, so it gets returned or parked until someone chases it. That can add weeks to your payment for a one-line mistake.

If you spot the error before the client does, don't edit and resend the same invoice number. Send a corrected invoice and say what changed ("corrected PO number, replaces INV-0217"). If the client never gave you a PO number, ask for it before you invoice rather than after the invoice bounces.

Numbering your own purchase orders

If you buy from suppliers, you are the one issuing PO numbers, and the same rules apply in reverse: your suppliers will copy your number onto their invoices, so it needs to be easy to match. If you are new to them, start with our guide to what is a purchase order.

• Never reuse a number, even for a cancelled order. Retire it and move on.
• Use leading zeros (PO-0001, not PO-1) so orders sort correctly in a folder or spreadsheet.
• Keep one sequence, even if several people place orders.
• Ask every supplier to quote your PO number on their invoice. Say so on the PO itself.

Our purchase order generator numbers each new PO in sequence, so the only rule you have to remember is the one about asking suppliers to quote it.